I have a kayak, a solo canoe and a solo raft/kick boat. I like all 3 for different reasons and waters.
The yak is a Tarpon 120 (12 Foot). It is a good size as a compromise between rivers and streams and 12 foot gives good tracking in lakes. My biggest concern is weight. You have to get the boat on / off the roof of the car or truck. Some yaks are big boats that claim their advantage is the ability to stand and fish. I can't see standing in a yak and floating a rocky stream of river, but more than that is they usually weigh a ton. The Tarpon was one of the early fishing yaks and still is very popular today with many improvements. It is very stable and I have never been able to flip it. It is a Sit-On-Top and that is another subject in itself. So the T120 is just my favoite yak. not the only choice.
There are several yak companies and designs to choose from and that is why you need to look around before you buy. Do you want to fish fresh water, salt water or both? Overnight trips?
Your size will often be a consideration. Just things to think about.
Why are you looking at a yak as opposed to another boat?
I would start by going to a site that offers reviews on just about any paddle craft. It is a good place to get the ideas and options flowing. Paddling.net has a lot info.
I will offer this, if you want. Come Spring, if you haven't purchased yet and you want to try one, give me a shout and you are welcome to try one or any of my boats to see if you like one better than the other.