What an amazing lens! It's fast ,being an f1.8, and crazy cheap too! These can be picked up on ebay for under £100 (the 50mm f1.8G is a little more expensive but I prefer the slightly older f1.8D model shown above). It won't autofocus on D3000 and D5000 series cameras though, or anything without a body mounted screw drive. If used on one of them, it would be manual focus only. This lens takes 52mm filters.

The large aperture of f1.8 makes it a great lens for low light photography and the shallow depth of field when shooting at f1.8 makes subject from background isolation extremely easy. The out of focus areas and bokeh this lens creates are beautiful. 

The autofocus is fast and snappy.  just a little noisy as it uses a body mount screw to drive it. 

While technically an Fx lens, it can be used on Dx bodies while the crop being equivalent to a 75mm on Fx.

For the price and ease of use, this should be in every photographer's bag, whether it's the f1.8D or f1.8G.