I already have the 18-200mm Sigma DG and was really pleased with the results so purchased the 70-300mm DG. These lenses are for my Canon 300d.
I am not a professional photographer but I am very happy with this lens.
Yes the motor is a little noisy but nothing major and it does "hunt" sometimes to focus but the quality of my photographs so far have been excellent.
I did read quite a few reviews saying that the 300mm end of the zoom gave soft photos, I have not had that problem, my pictures have been sharp and I have compared them full screen on the PC.
I was very impressed with the macro and amazed at how much detail it picked up at 300mm. On the subject of macro one thing I did notice was you do need to be a fair distance from the object to get it in focus (maybe 2ft+) where my other lens is much closer --within 1ft. It also requires you to select macro with a switch on the lens, this makes the lens extend even further making it quite long.
overall I think this lens is an ideal choice for telephoto and macro work, not that heavy and gives pleasing results. Professional photographers may not agree with my comments but I would recommend it.

I bought this lens to use it with my Sony Alpha 330 DSLR, as I wanted a telephoto lens that would fit my really small budget, and that would have slightly longer focal lengths than the ones offered by the standard kit lens sold with Sony Alpha 230/330 cameras (55-200mm).

After spending some time reading tests and reviews, I decided to buy:
Sigma 70-300mm f/4-5.6 APO DG Macro

This lens turned out to be a superb choice, especially if your budget is really tight, as it costs around £180 depending on source (I bought mine for £174 at WarehouseExpress).

Positives:

* 70-300mm focal lengths which is enough for an amateur
* macro switch for 200 - 300mm
* APO = extra low dispersion glass elements to limit chromatic aberrations
* price !!!

Negatives:

* Slow autofocus , which makes it bit difficult in wildlife photos, but you can still achieve very good results !
* Since the AF motor isn't ultrasonic apart from being slow, it's also noisy :)


I've been using this lens for over a year and I never had any reasons to complain about it. The quality of pictures you can take with it is great. I've been using it for taking wildlife photos (mostly at 200-300mm) as well as some random portraits using 70mm focal length (it does really nice job here!)
Maybe it would be nice to have the HSM engine for AF motor, but this probably why this lens costs only £180. I know that Sigma recently released a newer version of this lens, with HSM motor and optical stabilisation, but at the same time the price went up several times plus they have removed Macro switch from it !! (I cannot understand that, as I find this feature very useful and I tend to use it a lot! ).

This lens is a good choice and I would recommend it to all amateurs wanting some extra zoom, without having to spend hundreds or thousands pounds :)

regards,
bombalina