A friend of mine bought the Sigma 12-24 for use with his D100. The front element is indeed vulnerable as it exposed...They do provide some sort of barrel that attaches to the lens which permits you to use filters and all that..but it causes light fall off at certain focal lengths. One way you can address this is via the use of a spot ND.
Overall it is a good alternative for the Canon wide angles...it's almost the same price as a 17-40 here..

If you don't mind the vulnerability of the front element and also some flare issues, it's a worthy buy..
