I've got a 9-cell battery for my 620. As you probably know, it protrudes from the front (photo here or readily found elsewhere online) but it has not proven awkward in practice. I don't have a 6-cell battery to compare the weight/handiness to, but overall I'm quite pleased.

I haven't run it very low yet, but somewhere around 3 hours of heavy use (mostly non-3D games) only ran it down to about 35-40%; with a comparable C640 with two 6-cell batteries, 3 1/2 hours of similar use would have me at 10% across the pair of batteries.