This is a review from an enthusiast, not a pro. Pros please consult pro reviews!
I was tired of the crummy pictures I was getting from the D40s varizoom kit lens. If, like me, you're photographing family and friends 95% of the time, then a 50mm prime lens will change the way you take pictures.
As a portrait lens it is perfect - the soft focus results you get with wide apertures are far better than I ever got by using the long end of the varizoom. If you need to get wide, just stand back.
And if you're shooting in available light, which we all are, then f1,4 is a necessity if you don't want 5000 bland flashlight photos on your hard drive. The lens delivers beautiful contrast, razor sharp focus where you want it, and creamy soft focus backgrounds. I'm bowled over by the pictures I'm getting out of it. You will never go back to your varizooms.
Build quality: the lens is well made, compact, with a confident auto-focus (that is, it can keep up with my 2 year old, who is in perpetual motion).
There's a lot of direct comparisons with the (actually more expensive) Sigma 1,4. I also tried that lens in the shop, as the salesman assured me that it it was a better piece of glass, but I simply found it heavy and cumbersome by comparison. The Nikon lens is much more smaller, solid without being heavy, and keeps the camera light and quick to use.
Very, very much recommended.