I assume that's the same Sigma lens that was announced back in January right?
Sigma 18-125mm f/3.8-5.6 DC OS HSM: Digital Photography Review
I wouldn't say that is a useless lens any more then I would say the Pentax DA 18-55mm is a useless lens. I've seen plenty of great shots with "slow" lenses. If you just want a nice walk around lens with (hopefully) good sharpness with lower distortion than the 18-2XX's then that Sigma looks just about perfect. In fact I've been giving it plenty of thought ever since they announced it. I just want to see some reviews on it first. I could easily cruise around with that mounted on my camera 90% of the time then switch to a DA40 limited or DA70 limited when I have time take something important.