24-70 at 2.8 is a great focal length and aperture combination for portraiture. I normally buy Canon lenses, L series if possible. The 24-70L costs around £800.
After a couple of disappointments trying and failing to get one a bit cheaper on eBay I read up on Sigma's 24-70, it's a lot cheaper (£279 on the high street, less online).
I bought one just over a week ago and I'm blown away by it! It's amazing for the price. It's also available in Nikon and other fits. If you're looking for in that range I have to recommend you try it out.
I have a sigma 10-20mm and a 105 macro lens and as stated they are very good and half the price of the nikon optics that I mainly use with my D300 and D200.
I'd have thought that 70 mm at f/2.8 is a bit on the short side for selective portraiture Chris, but my brain is in full 36 x 24 mode. If you're talking half frame (say) then I'm with you.
Yes Tom, I'm talking about for non-full frame DSLRs, ( the vast majority these days) Often I've used a 50mm f1.4 prime and 105mm 2.8 prime for some of my portrait stuff but wanted the flexibility of a zoom lens.
I find the 100mm f2 canon has quite excellent optics, L quality, but it's not as robust, but for portraits, is stunning.
I'm on full frame, though, and value for money it is excellent....but not a zoom!!