You could do worse for auto navigation than any of the Nuvi models. The advantage of the 660 is the size, the FM transmitter and bluetooth. I really like it as a model, though if I was going to purchase one I'd probably go with a 370 for the European mapping, the ever-so-slightly longer battery life and the portability. But if I was going to be using a Nuvi in the car most frequently (as anyone who is hardwiring it in will be) than I'd definitely get a 660.
As far as using it in conjunction with a Powerbook... sadly, Garmin is still not Mac compatible, and that usually shows up when it simply won't see the unit via USB. Another caveat is whether or not it would like National Geographic or any other non-Garmin topo mapping. I know you can download waypoints, tracks, routes, etc... from the unit to N.G., but unless you have mad skilz you can't get the N.G. onto any Garmin product. The new Mapsource Topo 2008 is much better than the previous incarnation, however, so that may be the route to go. I loaded the earlier Topo mapping onto a Nuvi 350, so I know it can be done in one direction. If you were going to use the Nuvi as a receiver for your laptop (either with Parallels or a PC) you would just need to download nRoute from Garmin's site. I'm fairly certain it's free.
If you want a unit that's highly weather resistent you could check out the Zumos. They lack some of the cool features of the Nuvi but add some cool features of their own and they're IPx7.
