I had my Sigma BC1600 for about a week and the damn thing just quit working. New batteries in both transmitter & receiver, no luck. The transmitter just quit picking up signal. Even if I manually placed/rubbed the magnet right over the sensor.
I had a much older BC500 (wired) which worked fine but from my experience and now reading other problems with the wireless stuff, I cannot recommend Sigma.
If you do decide to use one, and you end up with a loose fit on the bars: They usually supply you with a small assortment of "rubber bands" which are actually just O-rings. You can pick up a variety pack of those at Home Depot for about $1, should give you more options. Too loose, just use a smaller O-ring.
(insert witty NASA/shuttle/O-ring joke here)