Wedged it in last night, it was a little tight, but after 5 or so minutes of a little gentle persuasion, I got my battery cover back on.
Performance wise, it's done alright, fully charged for 6 hours yesterday evening. Took it off charge when we went to bed and there was a 4% loss between 12:45 and 7:50ish. I've watched a few youtube vids, updated looked at facebook and taken a vid/some pics of my wee boy and it's currently sitting at 89% which is roughly what I'd be getting out of the stock battery. It's worth noting I've not had Wifi/3G on unless I've been on Facebook or youtube.
If it performs like this unconditioned, I'll be interesting to see what happens when it's fully conditioned. I've been impressed by alternative batteries in the past, only to see performance drop drastically after a month or so of usage.
The fact it doesn't really sit properly concerns me, but that could be unfounded. I am willing to give it a fair crack of the whip as it appears to have done ok so far.
Edit - 88% charged when first stuck in the phone.