There were a few models, esp., those from Sturmey Archer, that was not able to be used with any reasonable chainring/cog ratio withough risking damage to the insides, which made them only useful for small-wheel bikes like folders... but they have fixed that, I think.
Shimano 7-speeds have been decent from the beginning, even if they have a little more drag than a derailleur. Their 8-speeds were decent from the Nexus range, and the Alfine 8-speed is great (thats what I have). I have heard mixed reports about the Shimano 11 speed.
THe Rohloff 14-speed is almost universally loved by those who have tried them.
I don't know about Sram stuff, which was originally made by Sachs before they got bought out. I expect it is decent.