Do you use a HRM?

I don't use one. I use MFP's calorie counters for my exercise. I run and use a stationary bike. I also use HIIT training with my bike.

Yes, I do use a HRM.

Both MFP and the gym machines overestimate my calorie burn by 35-50%.

I actually just purchased one.. I figured I was getting serious about my fitness, and rather than hope that the estimates the net and MFP gave me were correct, I would know exactly what my body was doing.. I have found that some exercises were pretty close, but most were not..

It took a little getting use to with the strap, but now it isnt a big deal.. Its pretty quick and easy to put on once you do it a couple times..

I do. I use a Suunto M2 with a chest strap. I love it because MFP doesn't have the workouts that I do in their database (P90X) and since I've been using it I know what I burn, and also have come to learn just by how I feel what my heart rate is pushing so I "feel" when I'm getting close to my upper limit whereas before, I'd just go all out.

I teach Scuba diving so on a whim, I decided to see how many calories I burn (average) while teaching class. Suunto also makes dive computers and the watch says 30m/100ft on the back so I was pretty sure that would survive. I wasn't sure about the chest strap, but I wear it in the shower. I wore the chest strap under a thin wetsuit and the watch on my wrist. Basically, I was underwater in depths up to 12 feet ~ 3.5 meters(?) for up to 20 minutes at a time, and was in teh water for 2.5 hours. It survived and it continues to work fine a month later. Under the wet suit, it was subject to little water movement as it was tight against my chest, but there is more pressure underwater, so my thinking is that if it can handle being underwater, sweat, and showers it SHOULD be able to handle swimming.

Oh, and other than initially setting the date/time/your weight and fitness level all I do is clip the strap around my chest, put the watch on, hit a button and is checks for my heart rate, then hit the button again to start monitoring. No hassle at all!

I ABSOLUTELY use a Heart Rate Monitor! I love my Polar FT4... my first HRM ever. It makes sure that I really know what I am burning and don't over-eat because I think I am working harder! I also do a LOT of strength training, but I mix cardio in with it randomly. This tells me what I truly burned and doesnt leave me to an Inaccurate guesstimate through mfp... how can mfp know how much more I burn from doing random cardio in my strength training... or know how hard I work sometimes! I don't go to the gym without it!

Oh yes. MFP tells me I burn about twice as many calories as I do.