What HRM do people use.

Would an HAC4 or a Polar s520 be suitable for MTBing?

karl, might wanna scoot over to roadcycling for this. i might be wrong tho

Depends on how serious you are going to take HR training.One that displays the current rate and then stores the time spent in your training zones is what you want but I think most do that.I have a Sigma PC1600 and a Calvus but the Polars are quite good.

I tend to like the S720i.
Does most things, but you will have to adjust the range of the speed transmitter, as it tends to transmit a bit weakly for mountainbike use.
Good support. I *really* smashed mine in a massive crash, and had the bezel, glass and lcd screen replaced for about £30 at my local Polar service centre. I was well impressed at the toughness and repairability of the hrm. BTW the hrm was attached to the bars and took the full force of the crash.