If you open up Steam, go to View > Settings >  then where it says beta participation click "Change" and select the UI update.  
I think it looks good, and they now have a good place to show demos and videos, as well as some of the specials.

im in love with the grid view of my games.
 
they just need to let me add an image for some of the indie games that dont have one yet

DOUBLEPOST by DOUBLESHOCK.

Oh bullocks, you've posted twice!

yah, i like it sweet.

Pretty sleek. Gonna take a while to get used to though.

yeah when I posted, it lagged, so I opened a new tab to view the forum and see if it posted yet, but then it directed me to my topic, posting it again -_-
 
Anyway, I think it's good, but it keeps lagging for me.  I hope that's just because I'm loading pages that were never loaded before.

wow. it brings a tear to my eye. its so beautiful

I miss being able to see the fuckload of my games in a tiny font in a single list. Now I have to scroll too much to get a sense of what I have ready to play. Also the different tabs need WAY better game sorting options.
 
The in-game overlay is the shit though. 
 
Store pages need a wider background image than 1024 pixels. In fact the whole interface needs to be scalable, which seems much more plausible now in this type of UI.
 
Also, when listing the achievements for a game in the List mode, it'll have the pic [+1] instead of showing the final achievement itself. I understand showing [+2] and higher when you only have a single available slot, but when you do have that slot you should just friggin use it for the actual thing. (for example: select Audiosurf in the List view, and maximize your window at a 1920 desktop width, then look at the achievements in the info pane).

It's nice.  Looks a lot like the Windows products interface though, especially the Zune.

If it only worked. Had to reinstall to the old Steam. I thought the new one looked good, tho.

I haven't had any problems with the UI beta yet, crashing or anything some other people have. I really like it, and I think its an overall very slick, good improvement. It takes time to get used to the friends list and games tab being so different, and a part of me will always like the simplicity of the original 'My Games' tab and how you could sort and see everything easy, but overall I really like the new UI.

light-gray gradients everywhere are hideous (especially chat and channel window) and some areas of the skin take way too much space, ex. the bottom of the friends list and the banner of the main window (store, community) 
 
i used to use this skin before the beta, i find the overall look way easier on the eyes.
 
  http://rthaut.deviantart.com/art/Slate-Colored-v1-01-77575772

i dont mind it at all. the library browser with intergrated news is very smart. the ui we've had is like 6 years old which is incredible

Is there a way to see changes between beta client updates? I only see stuff for release updates in the News section.

I love it. It looks really great.