I don't know about the GM (doesn't seem worth the dl since Lion is just around the corner), but I've been running DP4 since its release and have no major complaints. Most apps work just fine, speed is decent. Animation speeds are mostly alright. On occasion there's some jerkiness but not with the frequency that it's annoying. I recently added more RAM and have noticed no animation issues at all since then. Swiping between spaces is quicker as well.
Printing is fine (Brother Laser), VLC, Movist, MPlayer all work. Evernote works nicely (except the Safari Web Clipper.) I've got Pages open all the time, that works nicely. Aperture is fine. Ditto for Sparrow. Reeder, Twitter, Time Machine, etc. All good.
If DP4 is this good (and I assume GM is even better), then Lion's debut will be quite smooth.
Pluses:
Mission Control: This took a little bit of getting used to, but combined with gestures this is most certainly the better way.
Spaces: Improved. Assign a wallpaper to each space, and assigning apps to a space is now much easier.
Resume: Excellent when doing work, but be careful during "entertainments." Otherwise I'd rather have it than not.
Previews: they can be invoked on virtually anything - icons, Spotlight results on-the-fly, etc. Love the functionality.
Preview App: This has been and continues to be a work of genius. Love the Fullscreen functionality with this one especially.
Gestures: they're there for nearly everything. Can't complain.
Launchpad: It's easier now, and actually more familiar to those who use iOS devices (these folks vastly outnumber Mac users.) For them, it'll be comforting. For the platform, it brings iOS and OS X together a little more (this is needed.) For me, it a) looks better, and b) it's easier to see everything and flip over a page than having to scroll via a Stacks menu.
Swiping between Spaces when Mission Control is not invoked: the cherry on top of the Spaces sundae. Brilliant.
Minuses:
Fullscreen apps are now off to the side, and each fullscreen window is treated as a separate app that occupies its own space. I'm still trying to wrap my head around this. Usability is not the issue. The issue is the logic behind this. I can't find any.
Not really a minus, but I'd like to be allowed under Safari to do a little more fullscreen tweaking: hide the toolbar and tab bar completely but show on mouseover. Currently, I have to selectively hide certain elements via the menu.
Let's see some more flexibility/options for touch gestures!
Mail.app: Great job bringing over the view from the iPad (but Sparrow has this too.) Great job with integrating conversations. So why is the rest of Mail such a massive cluster****? I'll stick with Sparrow on my Mac, and do what I've been doing for the past two months: use my iPad and iPhone for e-mailing almost exclusively. It's working.
My habits:
The most advanced thing I do is heavy photo editing in Aperture and occasionally Photoshop (I try to avoid the latter whenever possible.) Most of the time, however, I'm a Pages and (when I need it) an InDesign maven. I've never been a big fan of InDesign, however. It's a shame Quark was just left to rot, though I like to fire up the latest version from time to time. I haven't even tried it under Lion DP4.
On a daily basis I mostly use Pages, Safari, Reeder, VLC (or Movist/MPlayer), Evernote, Sparrow, and the handy built-in Dictionary app. Lion is great, but I find I'm using My Mac a lot less since I bought an iPad 2 around two months ago.
Specs:
Early 2008 MacBook Pro 15-inch, 2.4 GHz
Display (fwiw): Samsung 24-inch LCD
1920x1200 res (sadly, a rarity these days)
6 GB RAM (2 GB Ultra + 4 GB OWC)
Question:
SL had some font-smoothing issues as per here:
http://rubenerd.com/font-smoothing-snow-leopard/
What's the situation in the GM? Seems I still need to do the Terminal command in DP4.