Got my polarcell about a week ago. Seems pretty stable so far. I've removed my bus ticket battery padding and had no random reboots. I didn't see a bit jump in capacity at first. Upower was reporting 75% capacity on my 2 year old battery before I switched it, and the Polarcell started at 80%. It seems to have risen since then though, and it now says 86% of design capacity, 6.96806 Wh.

I can't objectively say it lasts longer, but it feels better.