Use MAC for 7 years and love its software, especially from Apple (except for iTunes). But Lion is definitely one of the worst experience in OS stability.
I was on Leopard and it goes for months with problem.
Stability
* The OS crash on me daily (more than once). Not consistently, but it does
** Crash when switching location and change WIFI network
** Crash when asked to go to sleep. You close the lid to go to a conference room for presentation, and it locked up -- that's a nice touch (even do that when I manually sleep).
** Crash when connect to external monitor
Usability
* Reverse in scrolling direction on the pad. That's is nice for the iPad crowd but is a huge time waster for older laptop users, especially if they also have to use Windows and Linux in their work
* The slow startup and open all previous applications - guess it related to keeping persistence for your document. This now feels like "Windoze"
* The aggressive text selector - try to guess what the user wanted and over select. Except the guesser is often wrong.
The ideas are nice, but execution is certainly not "Apple" like.