I haven't had the GX1 long, so bear that in mind, but I think its certainly a nice upgrade from the GF1. The extra pixels, the (much) better viewfinder option, virtual spirit level and improved high ISO performance all contribute to make it a worthwhile improvement. However I would say, that if you are happy with a G3 or GH2, then I don't believe that there is much benefit.
To a large extent it depends on what you like using. The GF1 was always my favourite Panasonic camera to use, and I've found the GX1 feels very comfortable and a pleasure to take pictures with. If you like that rangefinder aesthetic, and don't have problems with the external EVF, (very good by the way) then it may suit you.
Its a nice camera and it does what it does with little fuss. If you liked the GF1 I can't see how you could fail to like this, and it does follow in the footsteps of that camera very nicely, keeping many of the same virtues and adding a few nice little extras.
Hopefully, since Panasonic have finally come up with an EVF that works as well as their built in ones we can see developments in this product line as well as the GF3 direction. I guess the more they sell, the more likely that will be.