I had the DA* 50-135 before, but the niggling worry of SDM failure led to me to frequently reconsider keeping it. I also felt like I needed more reach for the sports and events that I shoot in college. I'd think that shooting full-body portraits at 200mm f/2.8 would give really nice backgrounds too.
When a great deal for a Tamron 70-200 came along, I immediately bought it and the DA* 50-135 was quickly sold. I haven't heard from the buyer since so I don't know if the SDM has failed on him yet.. I didn't have enough time with both lenses on hand to do a proper side-by-side comparison, but I haven't noticed any decline in optical performance in the field going from the DA* to the Tamron.
The screw-drive autofocus on the Tamron seems faster than the SDM, though it's less discreet. However, the AF whir has not given me any trouble even in concerts. Focus clutch is fiddly and engages with a loud clack.
However, the Tamron 70-200 is noticeably longer (especially with the deeper hood and tripod collar attached) and even though it's the lightest of the 70-200s it's still a great deal heavier than the DA*.
Lastly, the DA* has a much nicer zoom operation. It feels silky smooth when you turn the zoom ring. On the other hand, the Tamron turns as easily but just feels a bit rough.
I managed to get a photo of the two lenses together before I packed and shipped the DA*. On the whole, no regrets at all and I don't miss the DA* one bit, but that's just me.