My cat Manytoes has lots of extras, 6 fully extra toes, one with 2 perfect claws coming out of it (its a rear dewclaw actually, quite bizarre looking), and then he has an "extra" claw that occurs between 2 toes, but seems to be growing out of the space in between, it doesn't have its own toe. Behaviourally, he is completely normal for a cat, but I need to pay special attention to some of his freakier toes, as they grow thick and highly curved, indeed threatening his paw pads. My vet, knowing my stance on declawing, recommended removal of these claws, but MT is good enough with the clipping that I can take care of them. He is VERY GOOD at opening cupboards, drawers, and doors. He is also exceptionally good at "catching" like a football player when we toss him play mice!
I think his climbing ability is actually hampered though, his paws are so large and cumbersome with the extra toes, he cant "latch" on like other cats. Of course, this is also due to me keeping his claws very short!