My first gun was the G&G GR16 R5. I was pretty happy with it overall. I found the part(s) I liked the least about it was the hop-up. The biggest problem I've found with G&G's in general so far is their tendency to have annoying proprietary externals.
In the end (or actually a year later) I tore it apart and used it to build two completely new guns so I can't really comment on how it works with a few upgrades.