Nikkor 50mm f/2

General
Fast normal lens. Very cheap, very good. Is the absence of auto-focus a problem?

Pros/Cons
+ fast normal lens, excellent for low light or subject isolation
+ excellent image quality, as expected. Incredibly sharp and beautiful bokeh
+ very cheap - it's amazing how great image quality you can get with so little money

- manual focus can be challenging, but f/2 is better than, say, f/1.2
- tiny bit of contrast loss wide-open, nothing to worry about though.
- non-AI versions easier to find than AI. Make sure your camera is compatible (those Nikons without focus motor, such as D3xxx series, D5xxx series, D40, etc. can also take the pre-AI)

Taken at f/4. By that aperture, the lens has no flaws, really

Intended Users
Great for:

• people who look for a fast, quality lens, but are on a tight budget.
• great lens for a macro reversing solution - make sure its filter ring is intact
• low light or portrait work, this lens is in its element

Not for:

• anything moving - not impossible, but it can be very frustrating
• those who have a camera with focus motor - get the AF 50mm f/1.8 instead. Being able to autofocus is certainly worth the bit extra money
• too tight spaces/indoors. 50mm on DX is not great for small spaces.

Final Verdict
A great lens. Very cheap, very well built, and with excellent image quality. It's very cheap as well, which makes it a very attractive solution for people with entry level cameras who look for a (very) affordable fast lens which can deliver excellent image quality. Just keep in mind, however, that you will have to use your camera in Manual mode and guess the exposure.