Wow, still so much negativity towards Lion! It really surprises me; yes, there were some big UI changes, but I've adapted my workflows and now find that I'm more productive than ever. I thought no exposé would kill me, but actually the alternatives, so long as you have a trackpad (I have to use bettertouchtool on my magic mouse) the array of gestures and the level of control they give is fantastic.
I have Snow Leopard on an old Mac Mini that couldn't be upgraded, and the UI feels so, so dated now. I couldn't go back to Snow Leopard now I've learned to maximise the benefits of Lion. I also didn't upgrade the 2006 Mac Pro, but it's only used as a server. If I had a more recent Mac Pro I'd update in a heartbeat.
However, on old Macs that I did update I noticed a degradation in performance. I can understand how this would irk people. I'm fortunate enough that all the Macs I use at home and at work are within a couple of years old, and so Lion really shines. I guess if you've got an older Mac you'd be better off sticking with Snow Leopard. Perhaps Apple should have raised the barrier of entry for Lion and endured the wrath of its customers with older kit rather than allow the reputation of Lion, which is a fabulously powerful and stable OS, be tarnished.
I'm excited for Mountain Lion. I think Lion made great progress in moving along the desktop computing paradigm. Mountain Lion needs to continue this development while taking stock of the changes made thus far and improving efficiency wherever possible.