I upgraded to Lion pretty much as soon as I was physically able to. I think it has some great new features that I've been itching to get my hands on for a little while. There's a million and one reviews out there however, so I'm just going to share a few brief thoughts.
By far the most significant feature in my opinion is the sandboxing functionality and relevant APIs. Until recently Apple have had the benefit of not being taken very seriously as a malware target. The last decade has seen but a handful of 'viruses' for OSX. OSX also has a reputation for stability, with the majority of crashes being blamed on third party applications (such as flash). Apple's advance on the desktop PC market, as well as the mobile computing market has killed this comfort zone. With high profile Mac malware in the headlines of recent months it's clear that security is a growing concern - while the OS is still much more resistant to malicious code than Windows, there are always exploits. Sandboxing makes you and your data safer and any pesky third party instabilities are isolated as a side-benefit.
Full screen apps are probably second on my list. A lot of people have said to me "Windows has been doing that forever". In a way they are correct, however it's not entirely appropriate to compare full screen apps on the Mac with maximised windows on a PC. Partly because of gesture controls and the way that each application occupies it's own 'desktop' but mainly because of the way that the interface gets out of the way. Where in Windows the window periphery is just moved to the limits of the screen, in Lion everything from the menu bars to the dock are removed from view. The whole screen is given to the application's function and it really makes a significant difference. Safari in full screen is particularly nice, the entirety of the display with the exception of the address bar is webpage.
The other thing I feel is worth a mention is the Mail app. The mail app has long been a weakness in OSX. It struggled with formatting outgoing e-mails leading to pretty much mandatory employment of plain text and it was disappointingly basic in it's features. The real stand-out improvement in the new version is conversation view, where the mails ar parsed and presented in a compiled list. The app on the whole though is massively improved in almost every function.