This is the one place I use disposable batteries, Energizer lithiums. With them I have gotten well in excess of 50 hours recording time on multiple occasions. I know that means they do end up in the landfill eventually, but in my mind that's a small impact in exchange for peace of mind and convenience. By small, I mean smaller than regular batteries where it's touch and go after ~20hrs. YMMV
I have used Sanyo Eneloops before with the deck set to NiMH, and got 20 hrs or so before I started to get worried.
I used to be a strong proponent of the Energizer lithium AA's (and still am for many applications). However, the battery meter in the PCM-M10 isn't as accurate as the PCM-D50 and as a result, I've been caught out once by it. What seemed like a full tank depleted very quickly despite my best calculations as to how many hours those batteries had done.
Since then, as much as I keep tabs on time, I much rather charge up a set of Powerex NiMH batteries the night before a show. Then I'll plug them in to record the show, knowing I'm reading a fairly accurate battery meter and more importantly, knowing I'll get the usual 3 hours worth of recording that I need.
To be bitten in the arse once is enough for me. Otherwise, the lithiums are great and definitely long lasting. The best thing of all ... there's little to no risk of them leaking! That in itself is well worth the premium.