I think SDM is reasonably fast. The only thing is it doesn't feel USM fast/quiet. Back when I owned the Canon 30D and a few lenses (among them the 28-70mm f/2.8 L and 35-90mm $20 cheap lens) I never noticed how fast Canon USM was (b/c in reality it WASN'T "fast" compared to my 50-135mm). The big difference was was how quiet and accurate the AF was. My 50-135mm would occasionally hunt around in doors (even outdoors it is EXTREMELY occasional). Also, SDM isn't super quiet at all. It may be much more quiet than the scratching sound of screw AF, but compared to USM (even non ring USM), SDM is comparatively loud. Don't get me wrong, it doesn't bother me at all, but it seems strange that Canon has had this technology since the 199 something and Pentax is only just starting to use it and even this technology is inferior to the super old 28-70mm i use to have.
I still have no idea why Canon USM is so fast...
I have never had a real problem with SDM before, but I have had to service my 50-135mm due to a loose MF ring. I later found out you can tighten this yourself...
I would still recommend it. If you have never tried USM before, then SDM should seem pretty fast/ quiet.
The AF hunting could just be my camera body though... the 30D apparently had super fast AF system.