I think the general consensus was that it was nicely regulated, probably by voltage and not by current. Lots of threads if you do a search or look through quickbeam's review site.
On both fresh alkaline and NiMH cells, it appears to be equally bright, whatever that implies. I haven't seen any graphs using NiMH, but I've read reports here saying that some get up to 2 hours on NiMH.
I've posted this before, but it seems appropriate here. This is a SL Jr Lux on 2 AA NiMH (1800mAh cells), using Scopeview and a photocell. It shows relative output, not calibrated. I did it to see where it would drop out of regulation. As it says, it is 15 minutes per division, so almost 2 hours is practical.
Rob