Hi Phil, I have to Sigma 10-20 f4-5.6 and have to say the times I have used it, it has proven to be a superb piece of glass. It is quite heavy but that just gives me more confidence with it. Doesn't get used as often as it should!!! Is only made for use on crop factor cameras so in effect the 10mm becomes equivalent of 16 on a Canon, ie 1.6 crop factor. Can't say that I have had difficulty using it, but what you MUST ensure with a lens as wide as this is good foreground interest, otherwise you will find a lot of wasted space in your images and they will look very average. Mine was second hand from a friend so I knew its history,but there are very good deals to be had on used equipment if you go to the right places. But it is certainly a great lens to add to your bag, although it does have limited use.......  If you can get one at the right price, although new seem to have dropped a bit recently, I would certainly go for it. But as indicated above, it is NOT a lens you will use everyday.....

Like Dave, I also am the proud owner of a Sigma 10-20mm lens. In truth, I think it is easier to use than a long telephoto or zoom lens as it can be used with far longer shutter speeds without camera shake. Also, wide angle lenses also offer great depth of field. I love mine as the results are just great and most suited to rural and urban landscapes. 

The f3.5 version is a real asset in my arsenal and Philip if you can find a used one, either the F3.5 or 4.5 I doubt you will be sorry.

Cheers!

Rod.  

As Dave says, most 10 mm lenses of this type were developed specifically for the 1.6X crop factor cameras.  If you really need an ultra wide view, you have no option but to get one of these lenses. They are not cheap (typically £300 - £500 new). Personally, I couldn't justify the cost of such a lens for my 1.6X crop factor camera, because of the limited use I would get out of it, but this depends on what type of photography you do.

However, if you ever consider buying a full frame camera, it is much easier to get that ultra wide view with inexpensive used lenses. For example, I picked up a Tamron 19 - 35 mm full frame lens for under £100, which when fitted to a full frame camera, is equivalent to a 12 - 22 mm lens fitted to a 1.6X crop factor camera. This approach actually worked out cheaper for me because I had recently bought a used full frame camera.