I have used a HR monitor for a few years, but have recently bought a new (more reliable) one, I went out and trained at a steady pace based on my heart rate, monitored my HR closely and kept it within certain parameters and when I finished the 6.5 mile course was amazed to see I'd set a new PB without really trying. I guess I had run at a more constant pace than I normally do (I usually end up pushing really hard up the hills, of which there are many, and then my pace for the rest of the distance is obviously slower, whereas with the HRM on I eased off on the hills more than I would normally do and subsequently my pace for the whole distance was faster)
Moral of the story, if you have problems with pacing yourself, try a HRM it works wonders for me.