The Nex, the Olympus PEN, and the Panasonic GF-1 all fit into pretty much the same category. They are large sensored, compact, electronic viewfinder, interchangeable lens cameras. I have a GF-1 and I like it a lot. It's pretty small and has the image quality of a full size DSLR, a nice combination.
I don't know much about pricing, but I think this type of camera goes for a bit more than your $600 budget. I know the Panasonic goes for around $850 for the kit. The Nex is a bit cheaper than that with one lens. Unfortunately because it is so new, there is little information about the Nex. I certainly looks good on paper though.
If you are looking for something that is a little bit more than a compact type camera and would like the creativity and quality of an interchangeable lens camera then this type of camera is a good choice. But like I said above, I think you'll be saving your pennies for a bit longer, especially if you want more than one lens.