I had an RNS510 sold it because the audio specialists said for decent audio quailty you would need to change the headunit. Plus at that time the RNS510 didnt do full postcode search, so I took the plunge sold the RNS 510, i then bought:
£400 Rainbow ipaul amp
£950 on a Pioneer AVIC-F9210BT
£500 rainbow 3 way component speakers
alongside an 8 inch rainbow sub which i already had
It sounded brilliant however i missed the audio/navigation info on the highline, and it just doesnt look as good as the RNS510.
After a year i sold the amp, pioneer headunit and sub and ive now got an LED RNS510 . Ive lost a fair bit of money but im finally satisfied with my audio setup. Comparing the two setups yes the aftermarket route sounds crisper and bassier but in my opinion the money i spent didnt justify the ugly headunit (i thought it would grow on me) and the lack of compatibility with the car, it was too much of a compromise for sound quality. However i had to spend all that to find out what i really wanted
Go for what ever suits you budget and your audio needs. I personally will never spend another penny on aftermaket.
In my opinion you either have awesome sound with slightly compromised looks, or a headunit that looks great with slightly compromised sound.