I have had one for almost a year and I love it.
Pro's --- it has just about every effect under the sun...just about. Also you can download effects settings from Guitar World for your favorite songs.
The amp and cabinet modeling is awesome, it's like having a bunch of different amps available to you.... I can go from a Mesa Boogie rectifier
to a vox or a fender or Marshall or whatever...and you can hyper warp two different amps and cabs together as well.
The drum machine is actually pretty damn good, there's more to it than the casual player will find 1st try and there are so many ways to alter, plus you can always make your own patterns or download others too.
The recording features are awesome, but get even better using the ProTracks software. It's so much fun!
CONs- steep learning curve Digitech crammed everything but the kitchen sink into it, so of course there's a lot to learn, but even til you learn how to use it, just playing around with the factory presets is a blast by itself.
I could go on, but am at work and my day is over and I want to go home, but in my opinion the GNX4 is the best investment I've ever made in refrence to musical equipment and effects.