I have both fired a cannister of bear spray out my back door and have also taken an indirect hit from it myself (don't ask, long story - unfortunate accident). On both occasions it was this brand of bear spray.
http://www.udap.com/
I can tell you that from both experiences, this stuff works great. The time I got hit, I spent 20-30 min on my knees completely disabled. I would have bet a years pay that I would never see again. Honestly, at first I thought I was going to die and then I was afraid I wasn't. There is no chance that a bear or intruder would do any harm to you for a considerable period of time after getting hit with this stuff.
The time I sprayed it out my back door just to see what it would do, I was surprised how large the cloud of spray was and how far it shot. If you could get it out of the holster in time and get the safety off (same time frame as a gun), it would be almost impossible to miss a target (unlike a gun). This stuff puts out a huge cloud of pepper spray to at least 30-40 feet.
I have a whole gun cabinet of rifles and pistols (I am an avid hunter and gun collector) but I would rather have a can of this bear spray for both animals and intruders than any gun I own. The bear spray would definitely be more effective and easier to use properly. I don't see how you could possibly miss a target.