We've upgraded to 10.7 from 10.4 server - I did try an upgrade but that didn't really work so I started from scratch and used the e-mail clients to move the imap e-mail from 10.4 to 10.7 server. Running on a MacMini server.
So far so good we've been on it for a week its runnig DB server software, AFP, SMB - not used this a lot, VPN, Time Machine Server, Mail Server, DNS haven't quite got firewall there yet.
It also works as our fax machine with a Zoom USB modem - old school but converts our faxes to e-mail.
Like many have mentioned dislike the 3 server admin apps and it is dumbed down but it does work (so far). Using carbon copy cloner to make a bootable backup each day on a schedule. Having a minor issue on L2TP VPN but that was the same as on 10.4 - we were usng PPTP VPN before.
Also sharing 2x DYMO label printers I much prefer the way Lion Server shares printers - just like OSX client. For those that want to go a bit deeper point your server browser to http://localhost:631/printers/
Also sharing a firewre scanner.
Webmail looks good.
Seem to have a slight issue with the software update server it has yet to download the 10.7.3 updates and it would be nice to schedule when it downloads updates i.e when our office is closed.
Not sure if were lucky or if its just under used but pretty happy so far - faster than our 10.4 server. Can't decide whether to update the 10.7.3 update (currently running 10.7.2) I'm downloading the combo update but reading all these posts has made me nervous - never thought about updates on 10.4 just did them.
As for the criticism of documentaion non of Apple Server versions I have used have ever had any decent documnetation also so far I haven't had to use terminal which I did have to use on 10.4 - mainly to run mailbfr as in 10.4 Apple never provided any mail tools that actually worked.