Late piping in here, but just wanted to add my two cents. I've flown out of all five London airports and Luton is on the BOTTOM of my list. Besides the transportation problems already mentioned below, the staff seem to be inexperienced, rude and surly. I'd take the hell of Heathrow anyday over Luton!
A nice feature of Stanstead, besides the train station downstairs, is the fact that every airport bar has large plasma screens with the departure information. You never have to leave your bar stool to check the boards!
My favorite airport, hands down, is London City Airport (LCY). It's about 25 minutes from my flat by Tube & DLR! Unfortunately they only fly to a few close destinations in Western Europe; the runway is so small that only turboprop and small commuter jets can land there. I've flown to Amsterdam (£15 quid on VLM!) and Frankfurt from LCY.