Well, I got it yesterday :D
I couldn't really try it because the weather in London is really bad since the two last weeks (mostly rainy and cold) but the autofocus is fast and its macro feature works well.
I got it for the Canon 350D/Rebel XT. Next lens will probably be the Canon 70-200 f4 Sophia has so I have a range from 17 to 200 using those two lenses.
I do have the Sigma 18-200 that works well but it is more a "tourist" lens with photos not very sharp.

More about this 17-70 lens at http://www.sigma-imaging-uk.com/lenses/dclenses/17-70mm.htm

Hopefully some sun this weekend to test it :)

Congratulations Olly!!17-70 is an excellent lens for everyday use.Are you happy with the lens at low light conditions?i ask you because my sigma is very noisy,the most noisy comparing with the other lenses that i have.
Can't wait to see your photos.

I haven't tried it at low light yet but the Sigma 10-20 on the Nikon D70 focuses at low light very quickly and with almost no sound.

I didn't describe well,isn't the "noisy" the right word for it?I mean the noise on the photo which taken at low light and high ISO. :)

Yes, there is noise and noise :)
I have never heard of a lens giving more noise than other lenses. I think noise is related to the sensor only.
http://www.picturecode.com/noise.htm
If it was only the lens, you would have noise even at low ISO ;)

Nice article!After reading this,i did a test with the Sigma 10-20mm and the canon 70-200mm.Both lenses/camera were on the tripod,at the same distance,the same lighting and the same settings,i just changed the lens on the camera.
Both photos are 100% crop

Manual
ISO 1600
f 6.3
1/4 sec

this one taken with the canon 70-200
http://s12.photobucket.com/albums/a211/savra/?action=view&current=bbbIMG_7092-01.jpg&refPage=&imgAnch=imgAnch1

and this with the sigma 10-20
http://s12.photobucket.com/albums/a211/savra/?action=view&current=proto.jpg&refPage=&imgAnch=imgAnch2

I believe that the quality of the lens makes the difference and creates the impression of the worst noise.In my opinion the red and green dots on the photo taken with the sigma are worst and than these on the photo taken with the canon.

After reading this,i did a test with the Sigma 10-20mm and the canon 70-200mm.Both lenses/camera were on the tripod,at the same distance,the same lighting and the same settings,i just changed the lens on the camera.
But the two lenses do not have the same range so keeping the tripod at the same distance is not right: we are not seeing the exact same thing. On the photo taken with the Sigma you see a lot more dark area and noise generally appears in dark area while on the photo taken with the Canon we see the books closer and less dark area as it zooms more.
You should have tried with the Canon 28-135 which would be much closer (at 28mm) to the 20mm of the Sigma 10-20 used at full zoom compared to the 70mm of the 70-200.
Move the tripod until you frame exactly the same.
That said, I see a difference but really can't tell which is worse :D

You should have tried with the Canon 28-135 which would be much closer (at 28mm) to the 20mm of the Sigma 10-20 used at full zoom compared to the 70mm of the 70-200.
Move the tripod until you frame exactly the same

Although i'm not sure if i did it rightly,because when i moved the tripod the light condition changed a bit,as the camera with the canon lens was closed to the books than this with the sigma,i'm very impressed by the results.
The photo which taken with the sigma looks better and i didn't expect that! :p

Manual
ISO 1600
f 6.3
1/4 sec

Sigma
http://s12.photobucket.com/albums/a211/savra/?action=view&current=ed289f17.jpg&refPage=&imgAnch=imgAnch1
Canon
http://s12.photobucket.com/albums/a211/savra/?action=view&current=728372fe.jpg&refPage=&imgAnch=imgAnch2

I think it is hard to compare lenses in term of "noise" because I believe it is related to sensor only. That said, I am sure chromatic aberration (http://en.wikipedia.org/wiki/Chromatic_aberration) or lens distorsion have an effect in the impression we have about a photo, especially if there is already some noise created by the high ISO.
As the chromatic aberration changes all the time depending on what range you are using (10mm, 15mm or 20mm?) and on the aperture, it is really hard to compare lenses :)

I did take plenty of photos using the 17-70 today. I will upload some tomorrow.

That said, I am sure chromatic aberration or lens distorsion have an effect in the impression we have about a photo, especially if there is already some noise created by the high ISO

That's right!!can't wait to see the photos with your new lens!:)

I finally uploaded a few of them on pbase at full size before it went down. I will try later for the rest of the photos.
You might have to select "original size" below the photo to see it in full size. Be sure you are on a fast connection because each photo is around 5-6 megapixels.
I will take more photos tomorrow as I am visiting Brighton (on the coast). It should be sunny but we never know here ;)

At wide angle:
http://www.pbase.com/boxerworld/image/65790381
The same place at full zoom on the building. Very sharp!
http://www.pbase.com/boxerworld/image/65790801
Big Ben:
http://www.pbase.com/boxerworld/image/65791355
A monster :D
http://www.pbase.com/boxerworld/image/65791518
A little boat (full zoom)
http://www.pbase.com/boxerworld/image/65792001
London Eye:
http://www.pbase.com/boxerworld/image/65792120

Seems that is a very good lens!The details on the Big ben is so visible,the colors are vivid and it's impressive that there is no paramorphosis of the people on the the photo taken at wide ange!Did the images manipulated by a software or these are the original?

All my photos are treated by DXO in automatic mode as usual.
I think that Sigma is an excellent lens :D
You can read its review at http://www.photozone.de/8Reviews/lenses/sigma_1770_2845/index.htm

In term of MTF (resolution), if you compare it to the Canon 17-40 f4 L USM (http://www.photozone.de/8Reviews/lenses/canon_1740_4/index.htm) you can see the Canon is better at 17mm but at 24mm and 40mm the Sigma is better:
24mm in the center
Canon: 1923 at f4, 2004 at f5.6, 1987 at f8
Sigma: 1946 at f4, 2016 at f5.6, 1964 at f8
40mm in the center
Canon: 1832 at f4, 1981,5 at f5.6, 1915,5 at f8, 1825 at f11
Sigma: 1837 at f4, 2019,5 at f5.6, 1950 at f8, 1857,5 at f11

I am also very pleased by its low cost: the Sigma costs £237.99 against £533 for the Canon. A very good value for money :)

THANK YOU Olly for the info!i was thinking to buy the Canon 17-40,not soon,but it would be my next lens.I will try the sigma,seems it's a very good lens!:)

I did think about the Canon 17-40 because it is a full frame lens (unlike the Sigma 17-70) but 40mm isn't enough for me (I notice I am often at full zoom) and I found it too expensive for what it gives (but if you plan to take photos at 17mm mostly, then it is definitely the best).