Minolta AF 50 F 1.7 50MM with NEX-5
Don't get it if you don't have / don't plan to get a Sony Alpha DSLR/SLT body. For under $100 (vs. $275) you could get an old 50mm f1.4 manual lens (Minolta MC/MD Rokkor, Canon FD) and ebay adapter (SR/MC/MD or FD). The only disadvantage would be manual aperture (not really a big issue and not necessarily a disadvantage for MF), advantage would be much better MF with a wide rubberized and nicely dampened focus ring and real time DOF preview (manual stop-down). I have the Minolta 50mm f1.7 but use an MD Rokkor 45mm f/2 on the NEX, smaller and easier to focus.
If you want AF, the Sony 50mm f1.8 DT SAM will have some kind of AF with the new firmware but it will be slow.