X Rebirth came out on Friday, and it is facing the usual bugs that games from Egosoft suffer from at launch. On the Steam forums, they have listed the known issues, and some plans on fixing them. Essentially what has happened is that the game wasn't optimized for high-end PCs, so everyone with a decent gaming rig is suffering from terrible frame rates.
I started playing it on Friday, and I'll have a full review up soon. It's a big game, and I don't want to rush through it. The game is playable for most people, but if you are worried about bad performance and low framerates, you might want to hold off until a few patches come out. Egosoft is good at fixing the most glaring bugs quickly, and it's capable of smoothing out the bumps in its games.