I have used CapStun brand on an arrest scuffle and on dogs, and it was effective. Notably, the bad guy didn't stop and lay down, but it blinded him and interfered with his breathing enough we got control of him. (2 on 1 in our favor and he was winning - ah, youth). I have also been issued Punch-II and have personally purchased Guardian brand, all of which are quality products. Somewhere on the net I found a comparison of all the sprays and was surprised to see they really aren't all created equal. Do your research and buy quality. Incidentally, if you're going to leave spray in a car, I have found only one unit specially formulated not to burst - an auto unit made by Guardian.
The great advantage of pepper spray is that it's cheap and doesn't cause any damage. This means someone like your wife or girlfriend is much more likely not to hesitate, and will use it before it's too late. All tough-guy talk aside, most people have a tough time shooting a person and WILL hesitate, often because a lot of assault scenarios are not real clear cut until you're rolling around on the ground or have a knife pressed against your belly. A brandished gun is not necessarily going to stop things, either. The spray is intimidating to a lot of bad guys, can be used before things rise to the level of a justified shooting, and if they get it away from you and use it against you, it won't kill you like your gun or knife.
Incidentally, that Kimber Guardian gel-thing doesn't look good to me. The police unit has sights and is designed to deploy up to 21 feet (hopefully with other officers covering.) The civilian unit has no sights and takes away one of the pepper spray's advantages, a continuous stream or spray so you can hit your target. Check out the demo video on Kimber's website: their own demonstrator almost misses the head target with his first shot.