Indeed we are
OEM are definitely better for ease of use, it's just that there are some good aftermarket HU's not which are very user friendly, eg the Pioneer AVH2500BT I have works exactly the same way that the RCD300 did, has full touch screen, iPod menu browsing, and no battery drain.
To get the same functionality out of an RNS510 would cost more - I would prefer to have the RNS510 with MDI, and amp up all of the door speakers, but that's lots of extra money for a system that won't really sound any better
As you say there are lots of aftermarket HU's which simply don't work properly with the Mk5, or look really bad, or have annoying features.
I know very little about car audio to be honest, one thing I never really understand. I have a friend who has a small audio company who does a lot of my work :)