I have a slice battery for my T420/W520. I don't use it often, but it has come in handy a few times. It really adds to the bulk of the machine; with an X220 the extra bulk would be even more apparent because of the smaller size and lighter weight overall of the machine vs. the larger 14 and 15" T/W series cousins.
I've thought about getting one for my X220 but I haven't seen a need yet; before I get the extra battery I'd probably hunt down a Panasonic 6 or 9 cell main battery for it - since when I got it, it was equipped with a generic 6C which only holds a new design capacity of about 80% the genuine full charge capacity.
Slice battery is the only battery mine came with, and even then the runtime isn't that great, a rebuild is in order at the very least. But yeah, the extended batteries add alot of bulk, especially when trying to write or draw on the tablet. I'm curious if a ultrabase style battery might be less cumbersome than the slice, maybe someone on the forum has tried one.