I did a small stint in college working for public safety, and I talked about this very thing with some of the state police that I helped out during some events. The ones that have used mace in the past said that they would never use it again if they had their taser. The reason was that when trying to subdue somebody, you're not able to get a clean shot to their eyes all the time, and a lot of the time in the struggle they ended up getting caught in the some of the splash back. The stuff is so pressurized that there's a big risk of getting hit yourself, especially if there's a struggle afterwards. If you're close to the guy many of them have breathed it in after using it, and it almost dropped them. On top of that, it doesn't just burn your eyes it also burns your skin. When wrestling down somebody covered in mace you can feel it, and it's not fun.
I'll paraphrase the one officer who said it best: The people who would want to use mace are the ones who have never had to use it before, and just don't understand what can happen in such a situation.
After talking to them I've been dead set against mace.