Tamron 24-70 f/2.8 VC Issues - I Need Advice

How does the lens perform wide open at f/2.8?  I'm very much considering this lens, but, if you can't shoot wide open with good results, then it takes some of the "luster" off buying a fixed 2.8 lens.

I would be using this on a D5100, so with the crop effect, I would think this would perform very well.  And of course, if I go full frame at some point, this would still be a great lens.

Thanks.