We have the 640 as well as the optional XM (weather) antenna and the combination has worked well for us, using it pretty much as a "stand alone" unit. In other words, we never aspired to wire it in our former boat's 0183 network etc.
We've taken it of course on the road, but also is handy when you're chartering, as it's hit or miss on getting a chartplotter
and handy to familiarize yourself ahead of time on your intended cruising grounds.
Lastly, the XM weather has been nice to have - we can quickly get some bad summertime afternoon weather on the Great Lakes (as seen in this year's Mac race).
- touch screen interface to me is not as intuitive or efficient as a unit with some dedicated buttons / knobs
- have to purchase their blue chart g2 chips - haven't found a way to use the free NOAA raster charts (RNCs)
- not sure what is currently on the market in the same category, but the GPSMap 640 was introduced some time ago and may be getting eclipsed by more recent offerings.
How much integration are you planning into your boat's existing electronics setup?