I have a X220 with the 9-Cell battery. Overall I have felt satisfied with the battery life. I haven't done a full charge to death test yet, but antecdotally I think my system is getting a decent run off a full charge.
My question is with regard to standby.
I had a x120e with a 6-cell and with use I was seeing similar battery life to the x220. However, when I got the x220 I put the x120e away for almost 4 weeks.
When I was getting the x120e ready to ship (I sold it) I booted it thinking it would be long dead. To my surprise, it had a decent charge left.
My x220 on the other hand does not like sitting around. Even if completely shutdown it seems to burn juice.
This is bad for me, because I frequently leave it for a few days while pursuing my other ADHD interests.