I like the older ones better, primarily due to the different courses. For example, they took out an older course like Rye or something like that for the 3rd or 4th edition and replaced it with The Belfry.
I haven't seen the newest one, however, so I don't know which newer courses they've added.
I do agree with Sean, the color maps are terrific.
I have 4 or 5 copies, all different editions (mostly kind of beat up - guess it's not the kind of book you can buy and not look at all the time!
). I buy them frequently at our local Half Price Books when they pop in, usually for $5 or $6. In fact, they have an older right now, I was just there the other day, for $5.98. Let me know and I'll pick it up for you.