I asked a physician relative, and got some further info about the LED IR solution (this applies to all types of "clip-on" solutions). The main use is to measure oxygen saturation in the blood. It is able to register the pulse, but for rates above 110-120 it's more or less useless, so to get a pulse reading physicians do it manually, i.e. finger to vein and counting the beats over a timespan.
EKG's are much more able in the field of reading heart beats and behaviour, but it's of course way more intrusive than a clip-on, with all them electrodes and such. Finding EKG's off-the-shelf won't come cheap either, from what i've found. And having DIY things run a voltage through your body is usually not a good idea...
I'm going to build the LED IR thing later next week and see how it turns out. Who knows, it might be bad at reading the pulse, but perhaps one could do something fun with haemoglobin oxygen saturation...