I'd probably go for a -dare I say it?- Phatnoise. Of all the other existing soluting I think this is the best one. I especially like the way it integrates with standard HU's. Ok, so this is immediately also it's weakest point because that limits it's functionality, but I still think it's the best way to go.
Because I'm taking into account that within +/- the next 5 years there will probably be NO more new cars produced that support a proper DIN slot (it's already a becoming a pain to find one now). They'll all have their proper "standard" HU installed.
I think that's a good evolution for theft protection, but little else, especially not for audiophiles.