Its not the best 50mm out there, but its not useless, either. Just get a good hood for it (there are some threads on this already, a hood can really improve the quality of older lenses) and you're golden.
Lets list what its good for:
- Great for learning how a camera actually works, especially the relationship between aperture and exposure, manual metering
- Its great to learn about DoF, zone focusing and hyperfocal focusing.
- You can use it with cheap extension tubes/bellows for macro shots, because it has an aperture ring (which modern DA lenses don't)
Its probably not worth to sell it, but you can get a lot of value from using it