I just ordered one which should arrive tomorrow. I'll be 8 + 6 then so possibly too early to hear anything, but last time we were able to find DS very easily from about 10 weeks. I know there are mixed feelings for some about these, but I found it very reassuring and used it about once every 3 days to put my mind at ease until I started to feel kicks. I remember his heartbeat was around 180 to begin with, then dropped to about 140 and hovered there for the rest of the pregnancy.
Is anyone else already using one, or do you plan to?