I can vouch for the DICE unit, I have it in my car with a 2003 factory head unit. It's a little quirky at times, but 98% of the time it's great. I get in my car and my phone is paired with the stereo, I can play any music through my phone and the steering wheel controls work to change tracks and obviously volume. Calls come in and display the number on the stereo's screen, you can answer and hang up with the wheel controls. I love how my phone sits nicely down by the ashtray where it's easily visible, so dialing a number is reasonably safe with the gigantic numbers you get on-screen with an iphone. Sound quality is acceptable, but it's wireless so you get some loss. Installation was as Greg said, just running removing panels and harnesses no splicing or soldering or any of that. I put the mic up by the map lights, and phone conversations are clear and easy. I like that the mic is the only visible part of the whole system, it's completely hidden otherwise.
Negatives: the phone won't interrupt if you're listening to another source, i.e. CD or Radio. For me at least, the screen shows the name of the first track played, but never changes when the track changes, although I truly do not care about that function. Changing settings is a little weird, like programming a valentine radar detector if you've ever done that (turn on and press certain buttons within certain timeframes...) it's all in the manual and once it was set up I never changed it.
Overall I think it's fantastic, and about as good as any factory bluetooth integration I've used, with the exception of not interrupting the other sources.
If you just want an AUX in, there's a simple DIY for that and it's nearly free.