The manual is online: http://www8.garmin.com/manuals/GPSMA...nersManual.pdf
The 640 looks interesting, based on my quick read of that manual. Some comments/uncertainties:
1. Close to my personal sweet spot -- Can be used on our small sailboat for local summer coastal cruising augmented by laptop down below, and in the car or on vacations (portable). See also point 11.
2. Price point is up there - phew. Can get much larger screens from other manufacturers for less, but Garmin does make nice stuff. The price makes me think twice, though.
3. Can integrate XM satellite weather data.
4. Nice integration of AIS and other position data from a DSC radio, at least on paper, and presuming other people use their DSC radios.
5. Not clear to me how to network this device with a DSC radio and a wind meter, at a minimum. The 640 is NMEA 0183-capable, so is the idea to bus the 640 with other devices, presuming the 640 can supply the GPS sentences required by the other devices? Is NMEA 0183 enough these days, now that NMEA 2000 is out?
6. Screen is small, but I've been living with GPSMap 76C for years now, the 640 screen is twice the size, and that size of screen has found a big consumer automotive market, so it works for many.
7. Nice options for connecting to a computer for planning: SD card (though it is buried under the battery) and USB mini-port.
8. Short battery life compared to the 76C, so will need to ensure DC power is available.
9. I am not clear on whether the 640 can send position data to the DSC radio.
10. Garmin insists on calling intended tracks "course", and using "track" for "breadcrumb trail of past GPS fixes representing tracks made good", and using "heading" instead of "course being steered", but that's just my own pet peeve, shared by the tiny minority of consumer navigators, and I can work around it.
11. Speaking of consumers, this thing is a cool toy for the average recreational boater: decent marine navigation aid, but capable of good automotive navigation, displaying photo slide shows, and listening to XM audio by patching to exertnal speakers. They know their market.
On a serious long range cruise and bigger boat, I'd want a bigger screen and more chartplotting/networking capability, but I've got this product on my think-about list for our purposes.