I bought SW on Steam and it was one big buggy mess.
Fortunately some patches have been released and it runs much better now.

One big problem that remained is the opengl rendering issue with (all?) Ati cards.
Did they manage to finally fix this or are we going to have faulty rendered water etc again?

They can't really fix anything further with the ATi issues. It's a problem with the ATi drivers' rendering of OpenGL Buffers. Sadly, the only way it can be cleared up is with some ATi drivers that actually work with the game's rendering system. If you have an nVidia card, you shouldn't have any problems. I use an ATi card until my next GPU update (a ways off for me since I use a 5870) so I'm just going to pass on Stranger's Wrath for now, until AMD addresses the issue (and I won't hold my breath, either).