I have both a PB 15'' G4 1.5ghz and an iBook 12'' G3 700mhz.
The PB is my main home machine, and while I've thought about buying an external monitor, the screen is so lovely and bright, with the same effective dimensions as my old CRT 17'' that it just doesn't seem worth buying anything less than a dell 24'' LCD. So far, I've resisted that temptation as I couldn't really justify it.
I brought the iBook 12'' second hand as a backup in case the PB got stolen or broken, and also I was finding the PB just that bit too big and heavy to regularly take out and about.
I really like the iBook, as the small size makes a real difference, and also I can get it out in rougher places, where I would be worried about attracting too much attention with the PB.
Note: if your laptop is to be for road / cafe use, make sure you buy a secondhand one! Then you won't care too much if it gets stole / scratched / coffee spilled on it!
But, to be honest, I don't use the iBook as much as I thought I would, partly due to having a new baby makes it hard to get out much.
Another factor has been I brought a new laptop bag from Muji, the three way convertable one, which has been really fantastic. I used to use a Tucano Second Skin, and while it's good, it was a hassle putting the PB in it, then putting the Second Sin in whatever bag I was using, and extracting it when I wanted to use it and also finding somewhere to put the Tucano skin when I wasn't using it (it's similar to a dead whale carcass.. ).
With the new Muji bag, with special padded laptop compartment, [the PB didn't quite fit in at first - I had to unpick a couple of seams to enlarge it] I can just slide the PB in and out in less than a sec. Also the new bag is better balanced and just feels better. I take my PB out a lot more now, in greater comfort than I did previously.
I've now lent the iBook to my father to try to convert him

I'm not too bothered as its main function is is to be there if my main machine gets borked.
Just my £0.02
xoxo ..RedTomato..