I have been looking for half a year for a lense which could deliver very nice results for both portraits and very low light environments.
I had read very good critics about the Canon EF 85mm f/1.2L II USM and the EF 135mm f/2L USM, but for a beginner like me without any prior experience on using primes spending that big amount of money was not an option. After checking the very good critics on the reviews for the Canon EF 50mm f/1.4 USM, I rented one for the weekend and eventually bought one.
I got a good deal on a 2nd hand lense and since the moment I attached it to my camera, it has hardly been replaced for any other. Using a prime itself has refreshed my joy about photography. But the results that this lense delivers are what I was actually looking for.
On portraits you can get an incredible soft bokeh, and the colours out of it are very good. From f1.8 onwards its sharpness is also hard to top. Below that is a little soft, but I guess that must be the difference to the L-glass lenses.
For low lite environments (mainly concerts in my case), also superb. I can at last shoot concerts without having to use flash and thus getting pictures which do not catch the atmosphere of the moment or getting blurred pictures because of a slow lense/shutter speed. Most of the pictures are simply amazing, but you can often appreciate CA on chromed or metallic parts if shooting at f1.4-1.6 (again no L-glass; from f2.0 onwards problem solved) and the AF can be sometimes a little slow if the light is very very low (I guess it also has to do with using a 400D. It will probably be much better on a 40D or 5D).
I do definitely recommend this lense, since the only primes able to deliver better results are L-glass lenses, and that is simply a different league. If you are on budget, get it. No regrets.