when i first got my tamron 28-75, it's was such a joy to because the focusing speed was faster than the lenses i had. image quality is also very good. i guess u won't need to visit the service centre unless u dropped ur lens or something.
I used to be a 3rd party supporter myself.. that is until I bought my first Canon normal (100mm F2.8 Macro) and L (135mm F2 L) lens. The difference was big enough to get me hooked on original (unfortunately L) lenses. The difference is not so much in sharpness but rather in build quality (even compared to Sigma EX), AF (even compared to Sigma HSM), colour (my Tamron 28-75 looks flat, Sigma is yellowish) and contrast.
Don't get me wrong, I'm not saying that 3rd party is terrible and CMI but there is a noticeable difference although it might not always be worth the price premium. If you can't afford the originals, the 3rd party ones can still give you great results (I still regularly use my Bigma).
Actually I am not exactly experiencing any issues with my Sigma 24-70 2.8 DG MACRO except back focussing (correctable) and some inconsistency in focussing (sent back to Japan). Actually it isnt that bad- Maybe it is because I am particular that all my lenses focus very close to DEAD spot on.
But I read about soft image quality, lens fogging.... Frighten me only.
But Sigma lenses always seem tempting. They usually have what I want for a lower cost. I have yet to encounter paint peeling off (i dun wish to) so I still sort of trust Sigma. Just that after the hear say about inferior optical elements (I still doubt this).