It depends what you want from it - I am guessing things to do with people.
I only have two lenses - a wide angle, and an 85. The wide angle stays on the camera a lot of the time and it's pretty much all I use for my urban photos. The 85 is mostly on the camera when I am shooting people/street and it does its job well. For a crop body, which you have, the equivalent would be the 50mm, and when I had a 50 on crop I liked what it produced as much as I like my 85 now.
The main differences to consider between the two are sharpness and build quality - I find my 85 to be built considerably better than the 50 I had, and it also appears to be sharper than the 50.
If I was to find myself back on a crop sensor camera, I would be lost without a 50. Considering the negatives of the Canon 50, I'd look at the Sigma 50. I haven't used the Sigma myself, but if what I read about it on the web is true, it's a very good lens.
If I was to go back to a 50 now on full frame, I'd either get the 1.8 version or the Sigma 1.4. Canon's 1.4 is not enough of an improvement over the 1.8 to warrant the cost, and doesn't cost a great deal less than the Sigma
second hand.