I've just installed a Garmin Streetpilot 2610 in my '04 X3. It's far more user-friendly and offers many more features than the BMW factory nav. that I had in my 1999 740i. After using the 2610 for a month now, I definitely prefer it over a unit that is built into the car. The fact that I'm not locked into aging hardware that may become obsolete in a few years (as did my '99 740i nav. system), and the portability of a system that you can easily take with you when travelling in other vehicles are what sold me. But to each their own.

As far as the Garmin 2610 goes, it's a solid performing unit. By using a CF memory card instead of a hard drive it should prove to be reliable as well. The 2610 is recommended for use on motorcycles due to their added vibration (not good for a hard drive based unit).

To see some reviews of portable units there are a number of sites out there but one that's been reviewing GPS units for quite a while is www.gpsinformation.net .