You pretty much nailed the description when you called it a PDA + GPS + XM weather.
The display is extremely bright and have no problem reading it while on the move. I velcroed the unit to my dashboard for easy viewing.
Pros: no laptop required, takes up little space, easily portable, bright display, can run battery only. It's GPS+weather, displays and sounds a warning when you're in a county when warning is issued, easy zoom in/zoom out, can be set to overlay nearest metar data, cheaper than WxWorx, radar data is not smoothed, the $30 a month XM weather mariner package is plenty adequate.
I don't think I've ever used storm attributes/ cell tracking features while chasing, I keep it on "Nexrad" and "animate" mode.
Bottomline: works well and quite useful if you are like me and chase in different cars and don't like to be tethered to a laptop. For about $800 for the device, $30 month for data, I'm set! George Flickinger