No. If you have the funding for the Nikon I'd definitely choose it first. If you don't have the funds, you won't regret the choice to go with the Tamron. The Sigma I can't speak for personally, but if f/5.6 is good for you it does look beautiful from there on.
I want the lens to be sharp at 2.8... That's the point of buying a 2.8, haha. Anyone else have any personal experience?
no the nikon is not sharp at 2.8, but i doubt the others are either. although at 3.2 the nikon is acceptable and that's usually my limit for that lens. stop down more and it gets much sharper. IMO on a crop body, this focal range is absolutely the most versatile. on my d7000 this is overwhelmingly the most used of the bunch (we're talking like 80% of the time), even though it's the least favorite of all of my lenses. although to be fair the other lenses are primes or really nice sharp zooms (tokina 11-16, 70-200 vrii).