the heartbeat is a gross object, not very fast (may even get more slow than usual at some points), and not very conducive to refining attention and increasing the rate of perception. if you find yourself going the heartbeat route, try feeling for the movement of the blood throughout the body instead. this can be felt either as simple pulses occurring more or less at the same rate as the heartbeat does (then possibly diverging into greater complexity), or as fast vibratory rushes of fluid ricocheting through the blood vessels. i found these vibrations to be very prominent in my field of attention, and when i told one of the sayadaws in u pandita's tradition (with whom i was sitting at the time) about this, he encouraged my use of them as my primary object.

I would caution against using the heartbeat as an object based on a few small data points:

1) The end of the out-breath is when all things related to state shifts happen, and so using the breath as object somehow helps shift into new and interesting territory.
2) The breath, unlike the heartbeat, falls at the boundary of conscious and unconscious "control" in a way that few other things do, and so makes an unusually good concentration object.
3) The breath, unlike the heartbeat, can be felt in a large number of places, so if one doesn't work, you can try another one.
4) I know a few people who had strange negative effects when they took the heartbeat as object: one with heart rate issues as described above, one with very strange heart-area pains that persisted for a while after meditating.
5) No meditation tradition I am aware of recommends it as an object.

All those things being said, all sensations demonstrate the Three Characteristics, so technically any of them could be good objects, but that said, clearly there are better objects and worse objects, and, based on my limited data, the heartbeat is not a good one.