I've got one, make sure you get the APO one, which is sigma's low dispersion glass.
I used it on my 400d quite abit, haven't used it much on the 40d which I have now, sure it will be great, I have seen them under £100 2nd hand, which is a bargin in my opinion.
One complaint is to engage the macro function you need to focus all the way then engage the switch and same to take it back out, but I live with that.
At some point I will upgrade probably to something L series, but just can't warrant it at the moment.
The canon 70-300 IS is probably a nice bit of kit, but nearly 3 times tthe price 2nd hand.
There are some pictures on here peiople have posted using the 70-300 sigma.