It sounded kind of hokey at the time. (My father-in-law calls it "wuss cream"!) The sales lady said that she fell down some stairs and got some pretty bad bruising on her legs so she put some Arnica gel on it and the next day the bruises were almost gone. I was a bit dubious, but she was so impressed by it that I thought it was worth a try.
Boy, have I used it a lot since then! I think I use it on Creighton at least once a week; he's forever running into things or falling off things. And it turns out I have actually been really impressed. There have been many times where Creighton has gotten hurt and immediately after he's had a purpley blood blister looking spot - one that you know is going to be a nasty bruise - and I put the Arnica gel on it and by the next day it is practically gone.
One night, Stratton fell off the chair in our room, head first, and hit his forehead on the metal frame of our bed. Ouch! He had a deep line on his forehead where he'd hit (he was lucky the skin didn't break or that would have really bled!) and the beginnings of a nice goose egg. It happened to be right next to another bruise on his forehead that was at the green-almost healed stage. I put the Arnica gel on the new owie and the next morning there was no bruise. There was just a small bump and a faint line where the original one was. And the bump and line were both gone before the other bruise was. Impressive, eh? Yes, this one is a diaper bag necessity for my boys!
*Note: It works by dilating the blood capillaries to speed up the healing process. You're not supposed to take this internally or put it on any open cuts. Evidently it can be toxic, so use it carefully with kids.