NiMH rechargeables have significanlty less capacity than a decent alkaline battery. Although they should last long enough for a day's play in most cases, you won't get anything like the life between charges as you would between changes of a 9v alkaline. Consider also that many current paintball electronics continue to drain the battery very slowly even when turned off, so you must make sure you put freshly charged batteries in the day you intend to play.
Lithium Ion batteries have capacities much closer to that of alkalines, as well as being much lighter, so they can be a very good idea for powering loaders, although they're expensive compared to normal batteries.
Last note: be wary of 9.6v NiMH 9v cells - many boards can be damaged by any voltage over 10v, and since a fully charged 9.6v battery can exceed 10v initially they may cause damage which will not be covered by warranty. 8.4v batteries (the most common type of 9v rechargeable at the moment) should be safer, since even freshly charged ones don't usually produce as much as 10v.
Lithium Ion batteries have capacities much closer to that of alkalines, as well as being much lighter, so they can be a very good idea for powering loaders, although they're expensive compared to normal batteries.
Last note: be wary of 9.6v NiMH 9v cells - many boards can be damaged by any voltage over 10v, and since a fully charged 9.6v battery can exceed 10v initially they may cause damage which will not be covered by warranty. 8.4v batteries (the most common type of 9v rechargeable at the moment) should be safer, since even freshly charged ones don't usually produce as much as 10v.