I love my EF 24-105 f/4L IS. The only bad things I can say about it are:
1. Barrel distortion at 24mm is quite pronounced but fixable in Lightroom.
2. IS is not tripod sensing. In my early forays into landscape shooting with this lens I destroyed some great shots by forgetting to turn that off. Most newer IS'es are smarter than that.
I've never had any problems with flaring with this lens, so the same hood is fine by me. My experience has been that the wider the lens, the more flaring your are likely to get, so the better hood you'll need. Being a relatively slow f/4, I don't think it needs as good a shielding as the 24-70mm f/2.8 shot wide open.
I'm very happy with the sharpness, color and contrast from this lens. It's every bit as sharp if not better than my 24-70mm f/2.8 on my camera body (5D2). Some people (who probably have never owned one) might say that since it's a kit lens, it can't be any good. These people probably owned EF-S kit lenses (the horrible 18-55) and were burned by that.
I won't be rushing to upgrade my 24-105. I'd prefer Canon release an improved 17-40, and the updated the 35mm and 50mm primes. More than anything I want a compact full frame (5DIII) that has 7D-like AF and WEATHER SEALING! (Are you hearing me Canon?!)