I talked to a guy at the photo-store and he told me that the normal thing is to use the S2pro with non-rechargeable lithium batteries. When I nevertheless asked for rechargeable lithium batteries he seemed at a loss... That surprised me, and my question is : is it true? And if no, what do you use?
The S2 Pro that I have used on occasion works great with AA NiMH rechargeable batteries. Considering the cost of the Lithium it just doesn't make sense to throw that much money away, since the NiMH & charger pay for themselves the second time you change the batteries!
Of course the 123's don't come in rechargeable so you are stuck there...
NiMH cells work great in the S2, they can handle the high demand just fine. You could carry a pack of lithium batteries as an emergency backup since they have a much slower self-discharge rate, but I'd certainly recommend using NiMH as your primary batteries.
For what it's worth, p.112 of my S2 Owner's manuel says not to use AA Lithium or AA Manganese batteries - only alkalines or Ni-MH rechargeables. Not sure why that might be but ... At any the Ni-MN's last a good while.