While I've never purchased from them, MCE has been around for awhile and I haven't heard of any bad feedback for them. There's probably not a catch but I'd wait until some other forum members who do have experience chime in. You may also want to check with XLR8yourmac.com to see some of the feedback there.
I guess there is one catch: I think apps like iDVD require a G4 processor (although Apple's web site doesn't specifically say so as all new macs that can use iDVD come with a G4) so you'll never get those applications working on your iBook w/out a hack of some sort. Can't really think of anything else otherwise...