as someone who owns both the tamron 17-50 and the 50 1.8 I'll tell you my general use pattern. I love the tamron and it is on my camera about 98 percent of the time. What I've found, however, is that especially for indoor shooting, although f1.8 is valuable, you need a lot more working distance than can often be easily found indoors. Don't get me wrong, the 50 has its uses, but i would actually prefer to have something more like the sigma 30 mm f1.4, just for the extra width (and extra 2/3 of a stop!). I also use a 580 EX II and i find that it's much more useful than the 50 because i can use it with any lens, and it's a good pair with the tamron because if the light is somewhat dim I can get by with just the f2.8 aperture (which in my experience had been EXTREMELY sharp) and if it's too dark I can just add the 580 for help with AF and exposure. So, in summary, if i had to redo my lens kit, I'd go with the tamron 17-50, 580 EX II, and if i could swing it, the 24-105 L. Also I might get the 50 just for kicks, because it's still pretty darn cheap.