Canon 10D and Sigma lenses

[Sandpiper]

I would like to know of members' experiences using Sigma zoom lenses with the 10D. Looking at pictures posted in the gallery it seems that two lenses are regularly used, the 170-500 f5-6.3 and the 50-500 f4-6.3. How do these lenses compare to the larger zooms from Canon (albeit not in equivalent focal lengths)? My interests are mainly bird and insect photography. Which lens would be most suitable?
I own a Sigma 28-200 f3.5-5.6 Aspherical IF lens which is incompatible with the 10D, it will allow autofocussing but will not let me take a picture. Has anyone else had a similar problem?
Your kind help and advice would be much appreciated.

[helenh]

T'other half (paulh) had to trade in his Sigma 170-500 lens as it wasn't compatible with the EOS 10D (but it was an earlyish version of the lens). He's getting some good results with the Canon 100-400 USM IS.

[rayh]

I have recently purchased a 170-500mm and have had no problems with Auto focus apart from it hunts a little at 500mm in the A1 servo mode and I am more than happy with the results.

Regards Rayh

[138mph]

I am getting a 10D, I guess the excuse is bird/insect photography. I have used Sigma 135-400, Vivitar Manual focus 500 mirror, and the two Tamron XR 28-200/300 zooms , all with and without a Kenko MC-7 2xteleconverter.
The Tamron 28-300 has given me much the best overall results, but I have decided on the Canon 100-400IS, but am unsure on an appropriate companion lens with which I would expect to take dragonflies and things.
No doubt you will have guessed that I try not to carry a tripod unless I also have a scope with me (my current tripod head isn't upto a heavy load anyway), but I may get a monopod for use with the smaller lens.
Having bought the Canon 28-135 IS USM lens as I couldn't get quick delivery on the 100-400, I guess this post is in the wrong place.