They 'can' work quickly - as my dog has shown - however I am still using it and probably will still be using it off and on for a LONG time! The one I borrowed worked so well it confirmed to me that it was worth the money to buy one rather than curing the problem in the short time that I've had it.
YES there is a HUGE difference. In the 3 weeks I've been using one we were down to it being on rarely, however today we had a MAD barking session or two so it was on again all afternoon to remind her.
I firmly stand by it- if you can borrow one before you buy that would be great, but just don't get your hopes up for a couple of times only

Wendy