I love puzzle toys and am a big big fan of them!
Apart from keeping the dog occupied (it also tires them out) I have found that it increases the dog's problem solving skills and also increases the time it takes before the dog becomes frustrated. Of course, you start with a simpler toy first if the dog gives up easily but once you hit a certain level, I have found that they will just keep going and going. This translates to being more willing to try over and over again despite "failure" and really helps when during training of other things (tricks, etc).
Romeo played with puzzle toys from puppyhood and when he tackles the harder stuff now, you can almost see his brain click - and it's not just random pawing or just trying his luck to get something out. You can tell that he thought about it and would do precisely what is necessary to get the food out of the toy.
As an example, he once took a stuffed kong up to the top of the stairs and dropped it, causing it to dislodge food with every bounce on each step (not fun for me). And with one of the Nina Ottoson toys, instead of randomly pulling out pegs and pulling out trays, he would pull out the peg that corresponded with the tray below to get food out. I thought that was pretty amazing! I caught it on video here -
http://youtu.be/qh54n3qUdLQ
Love the Nina Ottoson toys! But a few of her toys can be DIY-ed at home with some simple easy to find materials.