I have a Nokia 6310i and the Jabra BT200. I use these in my car

When I first got it I called my wife and my answering machine at home
to see which was better - just the phone or the BT headset. Both my
wife and my answering machine tell me you can't tell the difference.

This is just in my car trundling along the A1(M) and the M1 at 70
(plus a little bit). I don't know how well it works in high ambient
noise areas or wind compared to just the phone, though.

I suppose it also depends on how noisy your car is! :-)

As a caveat, I use the 6310i with the leather belt holder, so the
phone is at my waist. After experimentation I use the Jabra BT 200 in
my ear on the same side as the phone. Under certain circumstances
(clothes material? weather? battery charge? who knows) your body can
mask the signal a bit, so you hear crackling and break up in the
headset once in a while. Having the headset the same side as the phone
prevents this, and you always get a good strong signal.