This Sigma doesn't disappoint in build quality. It's heavy, solid, tank like. Much like all the other EX's I own it's very well built. The hood attaches smoothly and easily. The lens cap goes on a touch harder than a Canon lens cap but it's not really even worth complaining about. The tripod ring is a nice feature to have included with your 70-200 and the carrying case is nice with a longer strap included for over-the-shoulder carrying. The 70-200 + 30D + battery grip is nicely balanced. Anyone who has concerns with the 24-70 in regards to weight will have issues. I think it's fine to handhold, no issues. But keep in mind, it's heavy and there's a lot of glass.
The zoom ring is silky smooth, as is the focus ring. Probably the smoothest of all the EX's I own with the exception of my 10-20. The 70-200 and the 10-20 both have buttery smooth action on the focus and zoom.
AF speed is quick. It feels equally as quick as my 85mm 1.8 when focusing from infinity to 1 meter at a distance of 1 meter.
The unfortunate part is I started communting a bit before 6am yesterday, and left the shop at 5:30 and had a working lunch. No ducks! So I was only able to snap a couple pics in my office and a couple while waiting for my truck to warm up. Being the rainy season in Washington it was dark as heck by the time I left. I'll try to get out during lunch today to find some wildlife. That being said though, my initial thoughts of IQ are very very good. I see very very good results wide open, at 70mm and at 200. I couldn't test a macro shot because I couldn't hand hold at 200 and zoom in to the 39.4" MFD very well. Macro is a bit of a reach, better to call this one "close up" with it's 1:3.5 ratio. Still, it's nice to use a 7x-xxx lens that is fast, sharp at 2.8, and internally focuses. I would say the 70-200 is sharper at 2.8 than my 70-300 APO is at 5.6, which really shouldn't be a suprise. Very happy with the quality of background blur also.
Very happy with the purchase!