I'm going to buy a kayak this weekend, it's something I've always wanted to get into, I love boating and being on/in the water in general (already have a canoe which I try to take out as much as possible, unfortunately you need a second person for that).

I'm wondering, how good of an exercise is kayaking? I know it will help with my upper body strength but does anyone know approx. how many calories I could expect to burn? I'm wondering if I can skip my regular workout on the days that I go out kayaking (I'll be shooting for once or twice a week).

Kayaking works more than just your arms. Proper paddling technique engages your core and legs too. It's great exercise! Like anything else, calorie burn will depend on duration and intensity. Paddling on a calm day is very different than paddling upwind and upcurrent!

I wore one of those sensewear armbands for a few days and went kayaking on one of those days. I did a 10 mile run, over about 3 hours at what I would consider a moderate / casual pace, and the device clocked me at burning 833 calories. So yes, it can be a pretty decent workout! For me, I figure about 250-300 calories per hour, but I'm really small, and bigger people will burn at a higher rate.

I usually go out on Saturday mornings (about twice/month), and I still do my early 5 mile Saturday run first, so for me, kayaking is "extra". But it depends on what you're used to and how long you plan to be out. DH and I are going kayaking in the keys over Memorial day weekend, and we'll be out all day long, so that's all I'll do those days.

In my experience you really have to push the speed to get a challenging workout, and at the beginning you may find yourself somewhat limited by your upper body strength (which will improve and then you can go faster for longer thereby keeping your heart rate up).

Personally, I wouldn't replace a run with a kayak, but I would replace a walk with a kayak. Like MBN, for me kayaking is just an "extra".

Good luck and have fun!