I have a Polar one now with a chest strap. I don't know the exact model, but it does give me a calorie count. You set it up with your age/weight and since it's getting constant HR info, I think it is a LOT more accurate than machines at the gym. Of course, nothing is perfect, but I feel like it gives me a good ballpark.
I had one before that didn't have the chest strap and you had to touch it to have it take your HR. It was less expensive and I thought it would be less hassle, but turned out to be so annoying! When I wasn't standing still, it had a really hard time reading my HR. It also started seriously malfunctioning (I'd take my HR and it'd say 47, then I'd take it 10 seconds later and said 189...really?!). So I'd say that investing in a good quality one with a chest strap is probably a good idea.
I like to know my HR when I run because like others have said, it helps me pace myself. It also tells me when I'm taking it too easy on myself
. It also has stop watch and other features that can be helpful.