I had been using the Sigma 18-125 for a long time, till I discovered the joy that is fast lenses. This lens is perfect for what I need.. Even at the same apertures, this lens is brighter than my old 18-125, which I felt was choking my 30D somewhat. I found that I was shooting at higher ISOs to compensate for the slow lens.
with the 24-70 f/2.8, this is not the case. I'm shooting at ISO 100 now, and loving it.
some people feel that this lens is slow to focus, and is loud.. Those people have never used this lens apparently. The focus is about half as loud as the Sigma 18-125, and maybe twice as loud as the HSM Sigma 70-200 f/2.8. It actually focuses as fast (if not faster) than that lens as well.
The ONLY gripe I have is, it seems to mis focus at 70mm once in a while..
I highly recommend this lens as a great alternative to the L version, and as a companion to the 70-200 f/2.8.