Deffo get a proper car kit unless you regularly switch cars. I have a Parrot CK3100 Evo in the Omega and a Nokia CK-7W in the BMW (both bluetooth). They both work great, the Parrot goes through the front speakers in the car whereas the Nokia has its own dedicated speaker.
Although I expected the Parrot to be better due to going through the car's speakers, I actually find the Nokia kit to be slightly better in terms of sound quality. It works fine with every phone I've tried, including Ericsson and Motorola, as well as Nokia. Another advantage of the Nokia kit is that it shows up on the phone as a "hands free" kit, rather than a "headset" which the Parrot does. No big deal, but you can have separate profiles for each on Nokias, and I want mine to be on silent (vibrate only) in all profiles except in the car. With the Parrot you can only achieve this by changing headset profile which means it also affects the profile when you have a music headset connected.
Personally if I were buying another I'd go for the Nokia CK-7W. It'd be much easier to fit in the Omega too, as the Parrot's interface to the front speakers requires bulky connectors to be plugged into the radio's speaker outputs, and it's bloody tight for space in there as it is. I had a right nightmare getting my head unit back in after installing all the gubbins; ended up having to cut some of the connectors out and solder the wires straight on instead.