I use mine on the 5D all the time. Like many others, I initially bought it as a normal prime for crop. It wasn't even my first choice, but after several disappointing attempts with Sigma's 30mm f/1.4, I found a used 28mm f/1.8. Glad I did. Modest price for great performance. Sure it's a bit hazy wide open, but stopping down 1/3rd stop is enough to get rid of that.
On full frame the wide angle/shallow DoF effect really shines though. I'd never used it before since that look doesn't exist on crop, but it does very well at f/2. Tons of vignetting though, but I usually leave it. Only really remove it even I need to crop a photo and I'd end up with uneven vignetting.
For a budget setup, you can't do better than minimalist kit of 5Dc, 28mm f/1.8 and 85mm f/1.8 and maybe a 50mm f/1.8 in between.