You really should offer two different GUI versions to download, one for Windows 7 and one for Windows 8.
Example: (Or)
If you run CCleaner on Windows 7 it should ask if you would like to enable Windows 8 mode or keep the Windows 7 mode. (Mode means the new GUI).
I think that bunch of CCleaner users might stop using CCleaner if there's no any chance to revert back to Windows 7 GUI, sad but true.
That would just end up making the file larger than it already is. The interface is pretty clean and not very Metro. It's a good balance.
Besides, people don't spend too much time with CCleaner to pay so much heed. For example, I just run it from the Context Menu of the Recycle Bin.
CCleaner is useful. People would hardly stop using it for something such as this imo.
Having just tried the beta, I have to say the new GUI looks pretty awful, especially when run on a Windows 7 computer. It looks completely out of place. I am at a bit of a loss as to why Piriform decided to change the UI, it's lasted this long because there's was nothing wrong with it in the first place?
TheAsian is right, flat UI's appear to be quite unpopular generally and I can't quite understand why developers try to flatten their software interfaces when the look is so disliked. It belongs on Windows 8 maybe, but certainly not Windows 7.
I hate the new GUI.
Lone Live CCleaner 4.x
Keith
FYI, It seemed to uninstall just fine.
I don't care for the look of the GUI but it's something I could get used to over time. What they've did seems cosmetic. My main complaint however is the color of the titlebar, it sticks out like a sore thumb!
However I'm thankful on two parts:
1. It works on WinXP.
2. The functionality and where things are located in the GUI haven't changed.
I noticed a fix they've did when selecting drives for Wipe Free Space since in version 4.x I always have to scroll the list, whereas now in 5.x Beta all of the available drives are visible without the need for a scrollbar.
In evolving times when Microsoft themselves, have started to phase out the Metro (ModernUI) - it is incredibly bewildering to find Piriform changing their once stunning GUI to such a retarded and hideous looking one. 
Not one to resist or hate change but the new UI is extremely poor - just like the whole "flat" idea. Stick to the old one and enhance it!
I noticed a fix they've did when selecting drives for Wipe Free Space since in version 4.x I always have to scroll the list, whereas now in 5.x Beta all of the available drives are visible without the need for a scrollbar.
They REALLY REALLY should have fixed that years ago,
when I complained that AFTER I had carefully scrolled and selected a partition for complete FULL WIPE (not just a wipe FREE space),
and when I then activated the WIPE the list scrolled back to the beginning,
so that I had to exercise faith in Piriform that correct drive remained selected.
Unfortunately my faith in Piriform was misplaced and a different partition with important files was wiped.
Piriform should at least offer 2 different versions. That's all I'm asking.
I have to say that the UI of 5.0 is really bad.
It looks like a child sat down and tried to copy what he/she saw, by memory.
It has been my experience that creators/companies that decide to "improve" the UI of their products rarely care about the opinions of their user community.
Complaining about it will probably not move them to reconsider the changes. In their minds the "more modern" design will always look better, just because they designed it.
Anyway, if the updated UI goes ahead, I'll likely stay with version 4 until I can find a replacement program.
I am a diehard Windows XP user, and the new UI looks so out of place that it would be difficult to continue to use it.
My plans are to build a nice Windows 7 machine, but it probably looks just as bad there.
Obviously, designed with Windows 8 in mind, and with little consideration for everyone else.
It's going to be a sad day indeed, when version 5 makes it's debut.
What bothers me the most (besides the flat buttons from Microsoft) is that the Title Bar no longer reflects the system colors. It's just plain white.
I have to say that the UI of 5.0 is really bad.
It looks like a child sat down and tried to copy what he/she saw, by memory.
It has been my experience that creators/companies that decide to "improve" the UI of their products rarely care about the opinions of their user community.
Complaining about it will probably not move them to reconsider the changes. In their minds the "more modern" design will always look better, just because they designed it.
Anyway, if the updated UI goes ahead, I'll likely stay with version 4 until I can find a replacement program.
I am a diehard Windows XP user, and the new UI looks so out of place that it would be difficult to continue to use it.
My plans are to build a nice Windows 7 machine, but it probably looks just as bad there.
Obviously, designed with Windows 8 in mind, and with little consideration for everyone else.
It's going to be a sad day indeed, when version 5 makes it's debut.
Very wise words my friend. Definitely agree.
I like the UI, but I guess for me it feels unfinished. I like the direction it's taking, but it feels like I'm using a mockup (which makes sense since it's a beta)
For example, the titlebar is much larger than the system default, and there's a lot of padding around the window controls.
I'll wait for the next versions and final release to formulate a more complete thought on the UI.
Also, does anyone else's UI not animate when minimizing?
Please Pirform don't use the flat tile interface. I hate win 8 and its stupid flat tiles. I like a more modern 3D style, not going back in time with flat tiles.
I think there is room for improvement of the GUI in version 5, but not by making it a flat tile.
There's no use in asking Piriform not to use their flat interface. They really don't care what you think about it.
In fact, I challenge any official representative of Piriform to make a post that says, that if enough users are unhappy with the Beta UI, they'll stay with the current one.
Could it happen? Maybe, but not very likely. They don't look at CCLEANER as a co-operative effort between creators and users.
I've seen so many companies go down the same path over the years, and there's no figuring why they do it.
WinRAR has done it right. They still have the same UI, same buttons that they had many years ago.
Users can design their own skins for the program and make it look any way they want. There's even a Windows 8 skin (although it's pretty bad).
There's no use in asking Piriform not to use their flat interface. They really don't care what you think about it.
Nobody knows what they're going to do!
I never remember them releasing a public beta of CCleaner before and I've been on here for 10 years, so maybe they wanted input from the community (without explicitly saying so) about the new UI design.
And I can't remember any company ever asking their client base what the product should look like.
It's always been, "Well, here it is guys - enjoy".
Only a handful even have the option of skinning.
Nobody knows what they're going to do!
I never remember them releasing a public beta of CCleaner before and I've been on here for 10 years, so maybe they wanted input from the community (without explicitly saying so) about the new UI design.
Can confirm; I've never seen a public release. Even Agomo was a private beta and you had to sign an NDA.
Every thing changes over time some for the better and others for the worse. More gray hear, different car models, new girl/boy friend(s) /wife/husband. Well you all get the picture .
But when it comes to software, things are a bit different in my opinion. There are only two ways to go.
Stick to what the developers provide and get use to it over time, which most users does or
the developers can implement Customization. Case in point Winrar, FireFox and couple more. But not all developers go down this path, could be due to security reasons or some other things.
CCleaner 5 I have to say does not look good in XP and even Win7 but I am going with number 1, why, easy as I do not look at the UI much, I look at the functions the software provides me to complete my tasks. Granted the UI does make the task a but easier .
But when it comes to software, things are a bit different in my opinion. There are only two ways to go.
1) Stick to what the developers provide and get use to it over time, which most users does or
2) the developers can implement Customization. Case in point Winrar, FireFox and couple more. But not all developers go down this path, could be due to security reasons or some other things.
I would suggest an option 3) the user has the choice to find another piece of software to perform the same function.
This is exactly what brought me to CCleaner 3 years ago when my previous crap cleaning software no longer suited me.