I got two and use one for zoom and one for crouch, so now I got all 3 main weapons bound to mouse buttons. I might get another for Use Item, cus I also have fastsky turn off with the same key, so it's nice for peeing through portals, but it's kinda wasted on Mouse3.
Anyways...works great. Feels way more natural than reaching for keys with my left hand. Does anyone else do this? If not I highly recommend it.