I have this unit installed now. Yes, you will lose the OBC. However, I really like the sound and versatility of this unit. The x-over system is one of the best I have seen. Really allows you to tailor the sound, and get rid of the annoying peaks. I initially had the PAC adapter that allowed you to keep the streering wheel controls, it worked, but there was always delay with the commands and the install is a pain if you want a complete integrated look.

In regards to the OBC, some of the usable functions, like the MPG monitor still work, but not much else, and there will be no changing any settings that are already in there after you remove the stock unit.

Personally, I did not care for the OBC. I do not use nor care about many of the functions it offered.

As far as Bluetooth goes, I have a RAZR V3x that supports the A2DP profile, meaning stereo audio via bluetooth.
Jensen makes a bluetooth module that accepts stereo bluetooth signals. 80$ at buy.com. I have that module connected to my aux input on the 9855. I customized the module's power adapter to work with 12VDC instead of 120VAC. This setup works flawlessly and sounds great.

BUT, as stated in the previous posts, give a few months. I enjoyed the tinkering but it would be nice to have a head unit with everything I needed built in. The 9855 isn't going anywhere, and at the very least it will be cheaper if you wait.