I wasn't expecting much, but had to buy it to test with anyway, since any new Mac we get at the office will have it. All my apps work (VPN, support client, Cisco VoIP, Office 2011, etc) which is obviously a good thing.
Being a heavy user of Spaces (can't live without it, actually), I was expecting to be pretty pissed off by how it's implemented here. However I actually like it better. It's very usable, very easy to move between spaces. How users of other OSes live without this is beyond me.
Speed seems to be on par, nothing extraordinary. However it's always good when it doesn't get slower!
Mail is fantastic; I use it instead of Outlook 2011 (which is buggy for me), and the new version is stellar. Address Book and iCal are kinda hideous, but I don't use them as much, so no biggie.
iChat is great, with the new unified window and tabbed conversations. Very handy when I've got group chats, individual chats, and different accounts all going at the same time. Cleaner, for sure.
Using Dashboard as a separate space felt kinda clunky, so I turned that off. Also turned off the resume feature, and switched the scrolling back to traditional.
One thing to note for corporate users is that since Lion uses SMBX (Apple's own implementation of Microsoft's SMBv2) Lion works much better in a Windows 2008 Server domain environment. Network browsing is improved, and all the machines show up across the network, without the need for a WINS server.