I shoot with a 300D and was looking to upgrade my lens kit (Canon EF-S 18-55). I found the lens kit is slow and I don't like how the front element rotates with focusing as I use the Conkin filter system.
I purchased this lens after reading many reviews. My choices were really between the Sigma, and the Canon 17-40 f4L and the Tamron 17-35. I finally went with the Sigma for a few reasons. Firstly, is well priced relative to the Canon and results on optical quality seem relatively close. The reason I chose the Sigma over the Tamron was range. The 18-50 is a great walkaround lens particularly when I'm not carrying my 28-135.
After comparing test shots between the Canon 18-55 EFS and the Sigma, here are a few of my conclusions:
1) I was quite surprised how well the Canon stacks up actually. Comparing shot for shot at equal apertures and focal length, the lenses were very close in terms of sharpness.
2) The Sigma 18-50 is very usable even at 2.8, giving it a clear advantage.
3) The Sigma is a "warmer" lens
4) The Sigma feels very well built and is nicely finished.
All said, I'm very impressed with the Sigma 18-50 f2.8. Hopefully I'll post some pics soon enough.