After having played with the Mac OS for some weeks now, I have to say that most of the noticeable changes are disappointing or insignificant, especially the most highly touted features.
My main disappointments revolve around the full-screen mode. It sounded nice. Truth is, it fails miserably with dual screens, which is nearly a necessity for prosumer and professional usage; suddenly, you have two screens to work with, for making notes from one to the other, or monitoring your email while working on a project--whatever possibilities are needed. It's why those professional stock traders are always shown with 6 screens or more. Even Apple's own external monitor shows it as a feature.
With Lion, you lose the second screen completely, it simply goes to the slightly weird linen background. That's not helpful; I added the second monitor for a reason, you know. I can see that the full-screen mode would be nice with the small MacBook Air, or a laptop with a small screen, but I own neither and I'm not in that "focus on one thing and one thing only" mode often. It's also a bit of a paradoxical thing, as traditional computers (the "trucks" of computing, as Steve called them) are all about true multi-tasking, while iOS is not. It sounds good to go full-screen and "focus," but then why not just use your iPad for that? There's a potential confusion here, making this highly-touted feature a bit unimpressive. I'm not saying it's not useful; I'm saying it's rarely useful to me, and I doubt it's limited to only just me.
The full-screen disappoints on another level, which is it's ineffective for large screens and text editing. If you go full-screen with Pages, MarsEdit, OmniOutliner (take your pick), suddenly your eyes have to travel a loooong distance horizontally. That's why newspapers use columns, as you can scan fairly quickly, right-right-down, right-right-down. Now, with full-screen on a 15" or larger screen, it's right-right-right-right x 20-down. That's a lot of work, and makes me tired and achey. Text editors need smaller columns on full, horizontal screens, with the menus vertical along the side (as with the iPad in a horizontal or landscape position). Otherwise, it's tiresome work. Again, this might be fine on a small MacBook Air or something, but not on a 15" MacBook Pro or a larger external monitor.
Mission Control is also lackluster. It's cute and pretty and fast, but not as fast as ctrl-tab to cycle through open applications, or LaunchBar (or another application launcher). Mission Control also hides applications that have been minimized to the dock, which was always my frustration with Exposé. I want all my applications available to switching, even the ones I got out of my way. To use Mission Control effectively, you must not minimize applications, and do full-screen instead.. This, to me, breaks Mission Control.
Finally, I find the animations in Lion to be annoying. I really don't need emails flying upwards in Mail, or pages turning in iCal, or pop-up windows really popping up. Just do your job, please, and notify my if there is a problem, but don't distract me when all is well. Just make the email disappear, and then do a "dwoop" sound if the email failed. That's all I need, thanks.
All that said, Lion is speedy, handles my external monitor better, and features some nice under the hood changes to security and secure memory. FileVault, the drive encryption option, is much much better (supposedly, that is; I'm still scared of it, personally). I also found the AirDrop feature for exchanging files to be very simple and easy, even though it's not as heavily promoted or reviewed as the other new features in Lion. I also really like the new design to the Mail application, it's minimalist but still very powerful. ICal also syncs with Google Calendar much more smoothly and quickly.
So yes, Lion is well worth the $29 upgrade. Just don't be too dazzled by the new features. The real killer feature is yet to come: iCloud.