Have to hand it to the software engineers over at Apple: They know how to slim down an OS. After years of operating systems growing considerable girth, Apple switched gears and put OS X on a diet. And it's a diet that Rush Limbaugh could be impressed with: after installation I went from 7 GB to 18 GB of available space! That was worth the price of admission alone, a paltry 30 bucks (meanwhile the geniuses of Redmond want to charge $200 to erase the memories of poor Vista victims). In addition to the hard drive space savings, there is a general increase of speed overall, the finder is significantly faster, and the scrolling through the dock icons is a breeze.
The best new feature is QuickTime 10, which does audio and video recording now, plus a really cool video screen capture that allows you to do tutorials and such. I plan on using it to show how to make some of the iPhone programs I'm developing.
My 2007 Mac Mini Core isn't ancient, but it's pretty old, so it took over an hour to install (a lot of sites were claiming 45 minutes) , and it's puny Intel Graphics card will not really be able to take advantage of the OpenCL features that would take advantage of any extra GPU cycles. Still, since the CPU has two cores, I expect video capture to go a little faster (haven't had a chance to try it yet.)
As far as compatibility with existing apps, I'm pleased to report that the GIMP, OpenOffice, and Flip4Mac (a .wmv viewer) are all working as before. Google Chrome, which is still in beta, is functioning but ironically Google's homepage had some strange hieroglyphics on it when I pulled it up. Most importantly to me, Xcode is working fine and it still recognizes my iPhone as a developer phone, which I was extremely worried about.
The verdict: If you have an Intel Mac with a gig or more of memory, this is a no-brainer upgrade. The hard drive space saved alone will save me tons of time every time I have to download the mammoth XCode updates, which usually require 6GB of free space that I rarely have. My Mac feels a bit faster, and I know I'll use the video screen capture a lot. This update should carry my Mac until the iTablet (if and when it ever comes out), and it should do the same for you too.