I've had an EOS 60D since March of last year with the EF-S 18-135mm f/3.5-5.6 IS kit lens. After 7 years of not having a telephoto lens (since my Pentax film SLR days!), I got the EF 70-300mm f/4-5.6L IS USM last week and have been absolutely amazed by its quality and performance.
I would like to look into getting an equivalent quality standard zoom lens to essentially replace the kit lens I have which has a tendency to be quite soft when wide open. Anyone have any recommendations? I really would like an f/2.8 and image stabilization is a must so from what I can tell, that really leaves only two options, the Canon EF-S 17-55mm f/2.8 IS USM or the Sigma 17-50mm f/2.8 EX DC OS HSM. The price of the Canon EF-S 17-55mm seems too high seeing how it doesn't have the build quality of an L lens and I've also heard it's not sealed well and has a tendency to suck in dust between the optical elements. The Sigma is priced very well and appears to have high quality optics, although its autofocus system doesn't seem great.
I've also never had any experiences with 3rd party lenses and am a bit apprehensive about it, especially since I will lose the excellent lens correction options in Canon's Digital Photo Professional software for pictures taken in RAW.
Would appreciate any feedback and advice anyone has.