I have had a Sigma 18-50mm f/2.8,the newer cheaper 18-50 f/2.8-4.5 OS and a Sigma 17-70 f/2.8-4.5
The 18-50 was quite good, but not at f/2.8, improved a lot down a couple of stops. The OS version I had was to be honest appalling and went straight back for a refund (just back luck I reckon) Last but not least the 17-70 which i was impressed with apart from it only managing f/2.8 at the very widest end.
The last one is now available in an OS version and despite the variable aperture could be worth a try at the price
I actually bit the bullet and bought the Canon 17-55mm in the end and it is a very impressive, but costly lens. One little niggle , you would think at the price they could include the hood
Just remembered I had a Tokina 17-35mm for my original Pentax DSLR and it was very good indeed, think it was around the f/4 mark, but can't quite recall. If their other lenses are as good would be well worth a look at what they have in the range you want