I have a nike one, I read a book by John L Parker called "Heart Monitor Training for the Compleat Idiot" and although I don't follow it's plans as such, the information I picked up from it is fantastic!!! It allows your training to be a lot more focused (in my opinion of course) and also allows you to monitor your effort levels a lot better - perfect if you're unsure of pacing in a race!
Just as an example, I had a PB of 42:50 which I achieved at Lincoln 10k 2007, a very flat and fast course. Not long after I got my Nike HRM and ran a 10k training run including a number of hills (one particularly bad), ensuring my HR stayed at a constant level (can't remember exactly what this was, but it was as recommended by the book). Anyway, considering it is generally deemed very hard to match race pace on a training run, I was pleasantly suprised to finish it in 42:33!! So HRM's get a big thumbs up from me.