I guess it depends on what you plan to do with it. I liked the water practice cartridges. For some reason, dogs find being squirted with water distracting when they are in chase mode.
The problem with any chemical agent like this is it works both ways. If the source of your vexation is upwind from you, your weapons is useless at best and may save your attacker the effort of incapacitating you. I've been sprayed with various chemical agents (military training and a jerk ex-roomie) and I can say from experience that it hurts. You really don't want to do much but get that crap out of your mucous membranes.
Whipping that thing out, particularly in iffy light could lead to an escalation you aren't ready for.
And finally, you gotta train with that thing. Figure out where you are going to put it for quick, easy access. The only way to find the right place is practice. If you are facing a human attacker, it has to be fast enough that you have it out and sprayed in their eyes before they have time to close the distance and beat you senseless because that is what they will do if you pull a can of Mace out and don't use it in time.
The last bit of your practice should be getting the hell out of there as quickly as possible. If the person recovers quickly, they are going to be pi**ed off at you.
re-cur-sion ri'-ker-shen n: see recursion