I have a macbook pro, which sits around and does nothing a lot of the time, and a macbook air, which I carry around with me all the time.
It annoyed me that I had to plug the macbook air into my 3TB external drive to back it up. Fussy me? Yup.
What annoyed me more is that it costs a fortune to buy a dedicated Apple backup thingy (the time capsule, I think).
What annoys me even more is that for the last year I could have been using OS X Server, installed on my MBP, for an all up cost of £13.
So ... I'm annoyed ... but very happy at the same time.
In short: I can now back up my Macbook Air, through wifi, onto my external HDD which is plugged into my Macbook Pro.