I use a 15" Lenovo and on the whole I'm quite happy with it. I like the fact that the keyboard has a number pad, and the screen is big enough that I can fit two

I use a 15" Lenovo and on the whole I'm quite happy with it. I like the fact that the keyboard has a number pad, and the screen is big enough that I can fit two Emacs buffers side-by-side without breaking up 80-char-lines. However, sometimes I would still like to have a bigger screen when programming, so I've just ordered an external 20" monitor. Portability with the 15" is acceptable, but you need a dedicated carrier bag, I wouldn't carry it around in my backpack. That would be the one reason why I might go for a 13" next time I buy a laptop - as long as I have the external monitor at home, the small screen doesn't matter too much.