Hey, I just received my Polarcell battery today, and not a moment too soon -- the only BL-5J battery I had left is no longer holding much of a charge, so I've been mostly N900-less for a week or so. Things look good so far -- for one thing, the battery fits snugly into the phone, unlike the "official Nokia" battery I ordered off of Amazon last year; no more battery rattling around in the phone! Once I get it charged, I should be able to get back to real coding again...

I haven't been doing any serious statistics on the performance of my batteries, but I'll try to keep an eye on whether the Polarcell has any significant benefits or flaws during usage, and report back here.