I've been pleased with mine. It's got better colour than my 14mm and at 16mm & 24mm, it resolved better than both my 14mm and 24mm primes, edge to edge. The only lens I have in that range that beat it was my 43mm.
The only draw-backs to this lens (my copy of it) is that it has a bit of zoom creep going from 16mm to 18mm, I can't use my singh-ray polarizers on it without vignetting at the wide end (Cokin size 'P') and while it's a very solid lens, it's not as solid as my old FA*24mm.
The advantages of this lens are that the SDM is very quiet, which I appreciate and it's weather-sealed (which I've tested quite thoroughly and been pleased with). Just for the weather-sealing, I consider this lens worth it.
My piece of advice is you go to the store and try the one you're thinking of buying out. There have been a lot of duds out there and it'll save you a lot of grief if you can confirm you've got a good one before you pay.