I just got the KDC-BT555U and I am not impressed. I still have stock speakers so I'm sure that makes a difference but it sounds worse than stock at lower to mid volume. I have the stock 7 speaker system btw. I plan to get some after market speakers when it warms up some so I will reserve my final sound judgment until then.
The other thing I really don't like is the controls and menu set up. The buttons are small and hard to operate in a bumpy riding jeep. Even the radio station buttons take me a couple tries to hit the right one. The main menu is accessed by pushing in the volume knob and turning to scroll through lots of options then pushing in again to select that tree, twist to scroll, repeat, then hit a back button to go up the tree and repeat. The whole time you are hitting bumps and selecting the wrong thing.
If you are the kind of person that can set everything up while parked and not need to adjust it on the go then it may work for you. Maybe it's just me and I should have got something simple with less features but I really wanted Bluetooth audio to stream music wireless from my phone. The BT audio sounds terrible IMO for music. My galaxy S3 speaker sounds better.
Good luck to you.