I have a 10 speaker GTI, to me no stock radio would sound good enough, so I bought an Alpine CDE-114BTi which is a 4x50amp with built in parrot bluetooth so I can make handsfree calls etc, and has a full speed iPod connection, so very easy to wire it into the glovebox and installed the radio along with a VW fascia plate.
It costs £180 which was a lot cheaper than any OE upgrade, particularly as it has bluetooth, and for Sat Nav, I use Co-pilot on the iPhone.
Whilst I certainly concede, the RNS510 looks a lot nicer as it sits nice in the dash, looks smart.
I bought an 8" Alpine sub for the boot, it's only small but even with this, the 10 speaker setup still doesn't sound great, it's good, but lacks thickness and warmth, the next step will be to upgrade the door speakers, I believe 6.5", a 4" and tweeter per door, so something a bit beefier should improve things if you fancy going down that route - although it would cost silly money for high end stuff ie Focal, a decent set of Pioneer speakers will be a massive improvement for a minimal cost.
If you stick a jumpstyle or hard dance track on, the doors will rattle into submission, but short of a full dynamat install, not much can be done about that.
So in summary, a small sub in the boot, along with upgraded speakers should see you with a good enough sound system.
I'll be doing the door speakers in the next couple of months.