which would you buy? not really interested in doubling the size of my n900 with the huge cap mugen battery, just want another stock-sized
battery at my disposal when travelling.

wondering if there is any difference between the mugens and the genuine nokia battery. i know mugen has a good rep

or are there any decent oem knockoffs for cheap on ebay that people can recommend?

I have the huge Mugen and a couple of stock sized Mugen batteries as well. I don't get the impression that they are all that wonderful -- well, the huge one is fun as a novelty.

I have had pretty good luck buying cheap batteries from Hong Kong.

If you buy stock size I doubt you will get better than the Nokia battery -- but you can certainly get cheaper, and quality is generally fine.

My current faves are labeled JAPOD for NOKIA BL-5J and they came in cute packaging

my thread also has some suggestions for battery's
Battery Tests and Phone MOD

I've had a Mugen for 3 to 4 weeks now and I am very pleased with it, I am finally able to use the phone on and off all day and not have to charge it before I go to bed, where as before I was having to charge it twice a day if I used it heavily.

The downside is that; yes the battery, cover is bulky but I am used to it now, the switch to operate the camera is difficult to use and rattles, I miss not having a kick-out stand and it also will no longer fit in fake’s laser cut stand

Other than that I would recommend the Mugen because it lasts forever.

I purchased a Genuine BL-5J from DealExtreme for $16 some time ago. It really is a genuine nokia battery, I tested it up and down, all the security is proper (dots from certain angles in the holo, etc). I have had a cheap knock off with fake holo before, and I can tell the difference. Also, this genuine battery works just like the one that came with my N900, it reports battery level properly on the phone, charges and discharges the same.

I was thrilled when I got the battery and it was legit, i also got a cheap battery charger from them, so i could charge one in the phone, and one on its own at the same time. It has been SUPER awesome having a second battery, im even thinking about getting another one. I would 100% recommend you get that battery if you need another one:
http://www.dealextreme.com/details.dx/sku.32444