I have a Nexus 5 on order, but my wife's Samsung Relay connects very fast and it is running 4.1.2 . My old Samsung Galaxy S 4G running 2.3.6 Gingerbread takes forever to connect and does not sync my phone book every time. Once I get my Nexus 5 I will update with my experience.
I just received my Nexus 5 last night and did not have much time to test but it paired up immediately with my Accord EX-L. Then I tested how long it took to get a connection, and that seems to be the issue mine took forever. I downloaded Bluetooth Autoconnect and played with it a few minutes and with that it connected within 30 seconds or so. I do not understand all the options inside of Bluetooth Auto connect so I need to do some more testing, but in my brief testing it seemed to work fine. Also, there is a patch coming out in the next day or so for Andriod 4.4 so keep an eye out for that and see if that helps.
Well, my Nexus 7 just got the latest Kit Kat update but not the Nexus 5? I did some more reading, a lot of people are complaining about Bluetooth connectivity to various car models so it appears to be a code issue on the Nexus 5.