I just picked one up last week for use with my 20D, and am very happy with it. I don't think that CA will be a problem for you, that's something you probably wouldn't notice on your slides. CA complaints are a lot more prevalent nowadays because DSLR users take pictures of backlit tree branches and check them at 100% view for problems.
The main complaints I've seen with this lens are from full frame users that dislike the barrel distortion and vignetting. Some complaints of flare are also out there, but the majority of that problem was addressed by Canon.
I'd say that you'll be very happy with the lens, provided the barrel distortion and vignetting aren't a major sticking point for you. Regardless, it'll be worlds ahead of your 28-105 f/3.5-4.5. If the barrel distortion/vignetting are major issues, and you don't care about IS, then consider the 24-70mm f/2.8 L.
Hope this helps!