One satellite short of a full constellation.

This ZUMO 550 is my second car GPS navigation system.  I briefly owned the 450, but then realized I'd spend more money buying the additional accessories (mounting brackets and cables) to bring it up to near 550 capability, so I sent it back for the 550.

The Garmin is well constructed and durable.  I'm on my second unit, as I had to send the first one back because it would often lock up and not turn on when going from one cradle to another (bike to car or vice versa).  The only way to get it to reset was to remove the battery for 10 seconds, then replace it.  This proved a huge pain when away from home when I didn't have the special allen wrench that came with the unit.  I tried a replacement battery, but it didn't solve the problem.  Garmin finally exchanged the unit and I haven't had any problems since.

My biggest complaint is that the unit is required to be in the cradle for the audio to work.  This is because the speaker is built into the cradle.  It would be nice to be able to use the unit without the cradle and get voice prompts or hear the mp3 player.

I'm also a bit disappointed in the navigation software.  My cheaper unit (Averatec) allows me to specify which roads to avoid in a route.  The Zumo's functionality is limited to hitting a DETOUR button to avoid the current road.  There's no way to avoid a roud later in the route unless building it with multiple via waypoints.

On the bright side, the display is very bright and works well in direct sunlight.  Few GPS devices can claim to do the same.  The regular software updates and availability of new map and POI databases from Garmin make currency fairly easy (but expensive at $100+ per release).  The rugged contruction has proven its worth on the constant vibrating motorcycle, and rain hasn't affected it.

If I were to change anything, I would make it so the user could modify the displayed data fields (like on my eTrex).  I don't need a speedometer on the main display, I'd rather have data such as total distance to destination or time remaining to destination.  And as with any GPS, the POI database needs more frequent updates.