I'm a bit of an audiophile, and whilst changing to a pioneer avh.
Id love an oem unit as it can offer all the creature comforts i need with the added functionality of interfacing with the car (mfd/mfsw).
I wouldn't be able to use torque obd tool with it (avh don't either) but i think thats the only limit these oems have. Except
The oem sound quality.
Ive heard (and would happily be proven wrong) that the oems have poor wattage output circa 20/25w. That wont do, even if it gives me a reach around after every drive.
My avh was cheap (actually free) as my mate gets it ex display, which i can ask him for more, and whilst the 50w is great, im not too happy with the rest of it.
Food for thought, the car came with a rcd300 and a parrot kit. The parrot kit connects to my iphone like a champ w bluetooth usb and qux. Sound quality was much better then the 300. Both items unwanted.