I use Rayovac Platinum NiMH "low self discharge" rechargeable batteries. They last as long in my rear blinky as alkalines for me. When I used to run an AA powered front light, I also used the Rayovac Platinum LSD batteries with good success. What others have said is true in regard to alkaline vs NiMH batteries in low drain devices such as blinky lights. The NiMH batteries are only of any benefit in such devices if they are the low self discharge variety such as Sanyo Eneloops (great choice) or the Rayovac batteries (good choice) I'm using.

Where I get the greatest benefit from the NiMH batteries is in my digital camera. I get more than twice as many pictures out of my Canon PowerShot with my 2100 mAh Rayovac Platinum batteries than with alkalines such as Energizer or Duracell.