I have the Sigma 10-20 EX/DC F4-5.6 HSM and love it. It's a super fun lens that performs very well on the D5100. The autofocus is fast, quiet, and accurate. It can be a little soft in the edges and exhibits some vignetting at the very wide end but that's pretty normal.
Of course, I would love to own the Nikon 10-24 but it is considerably more expensive. Overall, I'm happy with the Sigma lens.[h=1][/h]