Also, if you're concerned with charging your Droid while listening to Pandora, most decks that provide Bluetooth capabilities also have a USB port. You can use the USB cable that comes with the phone to charge it while playing your music.
I'd love to get a head unit with built-in BT, but it's a bit pricey to get a good one and I don't want to "settle" when it comes to my truck. The T505 has been great. Since I also drive a seperate company vehicle it's awesome because I can take it between vehicles and have my hands-free and music in whichever vehicle I end up in that day.
I use a cassette adapter in my van and a patch cable for my car (when not using my iPod via iPod control). I did find out that when using the cassette adapter, it does turn your stereo into a speaker phone (but if you have it plugged in but are listening to just FM at the time, you will need to disconnect your pohne or you will not be able to hear who is calling).
As for my car, I have a Pioneer deck that has the ability to upgrade to Bluetooth for an extra $100 (have not done it yet). I have tried hooking the patch cable up to the deck and it does work if I take my phone out of my case (plug does not go far enough in to connect with the case on). Overall, it is handy to be able to play Pandora through my stereo (especially my van). I have a long commute so it does get a little annoying listening to FM radio. So it is great to have the option.