Jeff B. wrote: "I am assuming the nimh reason is for recharging"
This partly correct. The NiMH rechargeables are really inexpensive if you use your light a lot. NiMH doesn't have a "memory effect" like the NiCad batteries, so... it is safe to charge them back up even if you've only used them an hour or so. When you leave them in a charger (like the Maha) that will maintain them at peak freshness until needed, then you are always starting out with completely fresh batteries.
But this is only part of the "bright light equation". NiMH operate at a slightly lower voltage then alkalines. NiMH is nominally 1.2v per cell where alkalines are nominally 1.5v per cell. So, in a 4 AA light (like the Tec 40), 4 NiMHs give 4.8v vs. 4 alkalines at 6v. However, NiMH can deliver way more amperage quickly. That is why they are so popular in digital cameras. A digital camera will just eat up alkalines. I know, I started my digital camera out that way. In a high drain device, the NiMH can actually last longer than alkalines (the digicam is certainly like that).
In a Tec 40, the KPR103 bulb is rated at 3.6v so the 4.8v NiMH AAs are "over driving" the bulb. But, this REALLY BRIGHT bulb in the Tec 40 is now a high drain device. So, the NiMH (with their quick deliver amps) are really allowing you to get to that brightness (it isn't just about recharging...that's just a bonus).
You are trading off bulb life for brightness. Like "louie" says above, "carry a spare". In another post, "Alan" reports getting 20 hr. bulb life from the KPR103 in a Tec 40 with NiMH. Your experience could be considerably less with any given bulb.
So...carry a spare ($1.29 at Radio Shack).
The bulb is easy to change in the Tec 40, but, you might want an LED like an ARC AAA or Infinity (Blue-Green) as a back up while you are changing bulbs. Plus, the Infinity (Blue-Green) is only "usefully bright" (not REALLY BRIGHT), so... it won't screw up your night vision or give away your position when you switch it on (not to mention 40 hr battery life on a single alkaline AA and a "bulb" that won't burn out in your lifetime).