I'll be honest, other than 1:1 Macro shots of LEGO, I don't use this lens often. That is a HUGE fail on my part. The Tamron 90mm Macro is a truly great lens and deserves better. Sharp, cheap, decent build quality. Without spoiling it; I recommend it.
So, mechanics, well, they are average for a 3'rd party lens. Tamron uses a clutch system that I personally really like. If you want to shift between manual and auto-focus, you have to shift the whole ring (like a clutch). Not as fast or as smooth as a USM lens from Canon, but fun.
Optics, because it's what you all really care about. Well, the optics are excellent. Sharp is not an understatement. Shooting wide open, it doesn't disappoint.
This lens kind of flies low on the radar of most photographers. It is a lens I would recommend over the Canon 100mm f/2.8. Mainly because of the price. The auto-focus on the Tamron is slow. Very slow, but when you are shooting macro shots, the AF really isn't that important.
So overall, it's a great lens and it's available for under $350 new. If you want a fast portrait length macro lens, this is the one to get.