The big deal with eneloops is that they don't drain. NiMH, and NiCad batteries will lose 70% of their power just sitting without use over about a month. Lithium does not drain, but has trouble putting out the amps that NiMH and NiCad can, though it does drain much slower than NiMH, and is much lighter. Eneloops take almost a year to lose 70% of power while idle. Eneloops don't currently have the capacity of NiMH batteries either, though they are close enough most people still prefer them.
I would home that MS was smart enough to use Lithium in the P&CK. Chances are you are using your controller enough that eneloops wouldn't make a difference. When they are nice, is in a camera. Ever put fresh rechargeables in your camera, take two pics. Then a month later go to use it, and it is almost dead? Eneloops solve this. But with new batts, and continuos shooting, good NiMH's will outlast Eneloops.
I can't remember the generic name for the tech Eneloops use, but there are non-sanyo brands out now that use the same tech, but they are $$$ too, and I can't recall their names right now.
TIP: Investing in a good smart charger, will with most types of batteries. My rechargables last twice as long now since I stopped using the free junk chargers that come with batteries. Free chargers usually charge until a timer goes off. Good chargers can do refresh (drain/charge) cycles, and charge until a certain voltage is reached (this prevents burning out battery). They can also do 'top-up' charge cycles to keep batteries full while they sit in charger.
~S
