I updated my 350d kit lense with the Sigma 17-70mm F2.8 and I haven't had the kit lense on since not once...
It's a good lense for price and seems to be visually clearer than the standard kit lense, also f/2.8 is a god send for low light situations, it also seems to excel at macro shots as well as you're day to day landscapes.
If anything the only critiscm I have of it (if it is one) is that it is significantly heavier than the kit lense which really isn't a bad thing at all, oh and if you have a filter set you will need to change it to 72mm...
Check my photoblog for some samples of images from it (link in sync), and don't be afraid to ask any questions if you are still unsure.
Russ