Davey mentions, The place where it falls down is resale value, build quality and future body support since new models wont always be compatible with 3rd party glass."
I have had problems with older Sigma lenses not being compatible with newer cameras (Canon) but, my 90mm f/2.8 AF SP Macro (which is the model prior to the present Di issue) works fine with my newest Canon cameras (7D).
I use my Tammy solely for macro and close up work and it produces very good to excellent IQ. Where I consider that the lens falls down is when it is used as a short telephoto in auto-focus mode. The AF is slow (despite the focus limiter) and tends to hunt. However, I have no problem with closeup and macro work since I primarily use manual focus for these types of shots...
Since I paid less than $100 USD for my Tammy which I purchased used several years ago, I have no complaints whatsoever...