Went from Snow Leopard to Lion on the day of release.
I lasted four days before reverting back to SL.
To be fair, I didn't encounter any of the (many) reported bugs in Lion - it worked just fine. However, the way that it worked was the issue for me.
You can find many, many other threads detailing people's disappointments and irritations with Lion, but in no particular order, some of mine were:
Colour scheme : Aqua has become a dull grey, which looks boring, and makes active / non-functioning tabs difficult to discriminate between. Not the end of the world, but a step back in my view.
Autosave / versioning : A real love it / hate it facility. Depends whether you want control over your software, or whether you want to relinquish it to the machine. Me, I hate it. Plus there are question marks over the performance impact of backups running in the background on large files, and how much disk space is being used for these backups.
Spaces vs Mission control : I wasn't a heavy spaces user, but if you are, check out the many threads regarding this issue. It appears to be a backward step.
Too many gimmicks : Launchpad, the screen animations, making the traffic lights so small as to be impractical to use, the iCal appearance. None of them dealbreakers, but they all ratchet up the annoyance factor a few clicks.
It's unlikely that iCloud will work with SL (maybe someone else can confirm or deny - I'm not 100% sure), but given that I like control over where my data is, I wouldn't use it, so that's no great loss to me. It might, however, be important to you.
I made sure I had an up-to-date bootable backup of SL (courtesy of SuperDuper!), and it's probably the most useful backup I've ever taken! I might take another look at Lion in a few versions time, but for me, the biggest impact it's had is to stop me buying the Macbook Pro that I've been speccing out for the last few weeks. I'll make my SL-running Macbook last as long as possible.
But definitely have a play with Lion before you upgrade, just to get a feel for it. You might love it, or it might save you a couple of hours restoring your old system...
