I researched this lens to death whilst looking for a good quality medium-long zoom. There are any number of options, and along with this lens, I looked at a number of others.
The Sigma 80-400 was one. Trying one, it had fairly good image quality, but the autofocus was painfully slow. Trying to capture moving objects was pretty hard work.
I also tried out the 50-500 - an impressive lens, but the image quality lacked a little something. I also felt that, when fully extended, it lost its balance - the lens became very front heavy.
The other obvious alternatives were Canon zooms, specifically the 70-300 DO IS lens, and the 100-400L. After some research on this site, and the excellent Photozone site (www.photozone.de) which provides lab tested resolution figures, I discounted the 70-300, mainly on a value for money basis. It's stats don't add-up for the £700 or so.
Which leaves the 100-400. This is a mighty lens, but having tried one out in the shop it felt a little light weight compared to the Sigma 100-300. Referring again to the experiences of others, I'd read that beyond 300mm it becomes pretty soft, negating the benefits of its extra reach. Looking at the resolution figures (Photozone again), the 100-400 doesn't really come close to the Sigma. And at nearly £400 more than the Sigma it just didn't seem to tally so...
I ordered the Sigma, and haven't looked back. This lens is a true gem - well worth the risk to anyone scared of moving away from the Canon brand offerings. The autofocus is lightening fast, and once locked onto a subject stays there. The overall feel of the lens is just right to: it doesn't extend when zooming or focusing, retaining a constant length at all times, so after a while you can judge the balance of it perfectly. Also, it's a constant F4 - F4 at 300mm is actually pretty fast. Oh, and did I mention the 5 year guarantee?
But the big thing with this lens is the image quality. In resolution terms, it's nearly untouchable. The resolution is greater than that of many more expensive lenses, including some of the Canon primes (the 300mm F4 for example). The images just have a 'punch' to them that you wouldn't expect from a zoom. In fact the only zoom I've tried which I'd place next to it is the Canon 70-200 F2.8 IS. My sister has a copy of this, and whilst it clearly has huge advantages over the Sigma lens in low light, it's impossible in good light to tell the difference.
Another hugely pleasant surprise was the discovery that this lens works with the Canon 1.4 MkII teleconverter - despite all of the guidance from both Sigma and Canon that it won't. I'll confess at this point that I initially bought a Sigma 1.4 converter with this lens, and to test my new found faith in the Sigma brand, it was completely pants. Very poor image quality, AF speed reduced, overall a pointless investment. However, I went back to the shop to see what I could get as a trade on the Sigma converter and decided to try the Canon one. To the surprise of both myself and the shop guy it worked a treat. There are obvious trade-offs in quality, but not significant.
The downsides to the lense? Well, it's very big and heavy - people usually look puzzled when you appear to be pulling some kind of heavy weapon from the camera bag. Some might also argue it's slow at F4. However, sigma have produced the 120-300 F2.8, and it's truly enormous, and has few other benefits over this lens. Oh, and it could have image stabilisation. However, I'm happy without it. Again, it would make an already large and heavy lense larger and heavier (and more expensive).
One final question is whether this truly great lens is a fluke given the superiority of Canon's lenses in most other instances. Possibly. There are lots of 3rd party lenses that aren't very good. I'd rather think, however, that in this case Sigma have got it right by keeping it simple and eschewing pressure to provide a more versatile lens. If the zoom range had been stretched further (70-300 or 100-400) the quality and AF speed would have suffered (though perhaps Sigma would have sold more copies). Instead it has an unfashionable zoom length, and rocks.
All of this speculation, and I'm not too worried if Sigma don't produce any more miracles - this one is enough. If you're in the market for this kind of thing, and are dubious about going for a third-party offering, this lens will challenge every preconception, and all for a very competitive price. A great value investment in view of the competition and one I don't regret making at all.