I'm using a Pentax Super Takumar 50mm 1.4 on a NEX 7, it's pretty good, fairly sharp wide open, it's one of the 'classics' I suppose, and it cost me about £80 near mint.
Only downside is the adaptors for slr lenses are bigger than the M ones, but other than that, you get exceptional lenses for a fraction of the price of M glass.