My only lens currently is the 50mm 1.8 MKII, and it's a great lens. But, it's a bit tricky trying to catch and frame shots quickly when I'm zooming 'with my feet'. Don't get me wrong, it's a fantastic lens, but a bit easier to use on portrait or even still life. Not so great, IMO, for catching candid shots.
I have been researching lens', and have been through most of the reviews here and on a couple of other forums. For my purposes, I don't think I'm going to do much better than the Canon EF 24-105mm f/4L IS USM
What I like and dislike about the 50mm is illustrated in the shot I have uploaded here. With the low f-stop, the exposure is pretty good with ambient light, but I have some difficulty framing the shot quickly due to the lack of zoom. This is especially tricky when children are the subject.
The shot here was shot at:
1/250s @ f/2.2, ISO 400 FL:50mm