Hey mate,
Ive used a few different options both at PI, broady and Winton.
By far the best is the racechrono with the large screen you can purchase from roost industries
http://www.roostindustries.com/gps_lap_timers.phpI've tried the old phone with the bluetooth receiver and racechrono but found it was to inconsistent sometimes it would work no probs sometimes it wouldn't. I think part of the problem was the battery terminals momentarily losing contact and it dropping out but tried lots of ways including duct taping it up but it was still to random for me to be happy. I'd come in from a session thinking it was a good one and their would be no data.
I eventually relented and purchased the screen. I have it mounted on a bracket just inside my fairing and when I go down pit lane I just flick it across like unlocking a iPhone and I can see my laps while riding, it flashes up green if you go quicker through a split.
You can then check all your data, overlay it on someone's lap that is slightly quicker and see where you are losing time etc.
Don't get one that requires a beacon as you basically can't use it anywhere but a race meet.