SWD and Beast pretty much covered it, but count me as another DO IT vote. I fish out of my kayak more than boat. I started in 2008ish with the cheapest 10' Pelican I could get my hands on at the time. Since then, I've fished a 14' for years and now fish a Jackson Cuda 12'. To me, 12' is the perfect compromise of tracking and ease of use, etc. Like Beast mentioned, make sure you have a good seat or you won't have a good time. There's nothing like a big fish hauling you around in a yak.

ETA: Grilling some kayak caught trout right now actually.