I mentioned in a thread a few years ago that I have a G6 and that's one cart that I get PMs about asking if I'm selling it all the time (I'm not selling it, BTW). They seem to be desirable.
The plug's drivers work on Windows 7 and it's a great flash cart. It has a limited size though, as in it's not expandable like the EZ Flash IV where you can swap out the Mini SD card if you want.
As for how much you'd spend? They sold for something like $60-$90 new as far as I can remember. I got mine on a 'we need to get rid of stock' sale for ~$30. But GBA carts that aren't EZ Flash IV's seem to be quite rare these days, or at least aren't on the market as much. I would offer $30, but be willing to go up to about $50. If you find you don't like it, you can always sell it on the Trading Forum. They're rabid for old GBA carts there.
EDIT: I also wanted to say that, of all the flash carts in my collection, this is the one that has seen the most use. It's my preferred card in my collection.