I think it's very much a personal preference/threshold issue. I tried a 17" MBP for a while and got sick of carrying it around after about 2-3 months and I really didn't use it again after that.
Having said that, the new 17" is really tempting again because of the resolution. But I have to keep reminding myself that last time I fell for a 17, it didn't work out so well.
But again, I can see how other people love them. It is very nice to have that kind of real estate when working. It's just not for me, I guess.