I don't have the 10-20 but a tamron 11-18 so maybe not that useful an answer, but here anyway!
These wideangle lenses take some getting used to - initially you get too much sky or pavement as its a shock to see that much. When you realise that you need to be far closer than you'd ever imagine you are getting there.
I love mine - when I want that effect. Its a fun thing to do and when I put it on the camera it tends to stay on for a while. Sometimes the angles, especially of buildings gets to me.
If you can afford a fun lens, go for it. Does it fit with the other lenses you have?