I've recently (last week!) bought the Sigma 70-200mm f2.8 II and can totally recommend it.. especially at roughly a 1/3rd of the price of the Nikon.
Build quality is excellent, fast focusing, has a 'macro' feature (very usable close focus).. produces nice bokeh wide open etc.. I'll be using it in anger at some gigs towards the end of the month, taking some band portraits etc.. I might even be brave enough to post some on here for C&C..
Obviously, like most things, it depends on your budget. Of course the Nikon will be best, I'd expect so for the price.