I doubt the sigma 10-20 could EVER be called "distortion free" it is certainly good distortion wise for a lens that lives in the focal length of what is usually the land of fisheyes. but be prepared for giving your images some serious torturous work out in Photoshop's lens correction plug in
looking at this early review I get a sense of deja vu from when I first tried using mine for shooting buildings...
http://www.ephotozine.com/article/Sigma-10-20mm-f4-56-EX-DC
it's good for getting a panoramic view from even just a few meters away from your subject and for getting shots where you just can't get far enough back.
It's cheap enough to buy with pin money so I'd say get it anyway if your using a 1.6 crop frame camera.
but if you want to do architectural imaging as a serious hobby or job then a tilt/shift lens would be the way to go.. not cheap but good specialist lenses rarely are.
that said I'd be having manual focus driven anxiety attacks trying to use the Canon TS lenses on anything with out a BIG BRIGHT eye piece, a grid focus screen and full frame - a 5D or any of the 1Ds's... on a 300d it's a one way ticket to la-la land trying to manually focus or even getting your tilt right