Got a dodgy copy, don't seem to be isolated in getting one either. Your results may vary. It's always been decent enough in low light though, same as with any 2.8 lens. If you're shooting people running about you'll need to be faster but it's fine otherwise.
But yeah, not a fan of Tamron really. Their quality control is really bad. If you get a good copy they're very good value, if not they're a huge huge pain.