I use one for DD1. I have been using it for a few months so definitely younger than 3 (she isn't 3 yet) and she definitely understands the actions and consequences thing so I have found it pretty effective. Yes it has had a positive impact.

I use this as well as time out (the thinking corner) because they are for different things. Time out is for pulling her out of a situation where she is doing something unsafe, or repeatedly doing something I've asked her not to, hitting or biting etc. Where as the reward chart is for positive reinforcement for good choices i.e. listening, putting away toys, playing nicely with DD2, getting ready quickly. Also time out is mostly likely for things like hurting some one (i.e. biting mummy) where we can sit and talk about WHY we shouldn't do that and how it makes mummy feel, where as the reward chart is more for things that don't have such a tangible consequence that we can talk about IYKWIM and are more for 'nice to have' things.

At the moment we've got a packet of stars stickers that are all the same so no deliberating there! We have different rows for different actions and when she gets 10 stars (10 from any row, doesn't have to be 10 'putting away toys' stars) she gets to pick a treat. We generally point out when she can have a star but she also sometimes tells us she should get a good listening star She is so funny, she has NO imagination when picking treats so she just says going to subway every time - we are trying to expand her treat options LOL.

I think for the first 10 stars we made it pretty easy to achieve so she got the first reward and got what was going on. Then after that we tightened up the criteria a bit!