I've had one fitted to my ST for around 5 years... I swear by it! 
Just wish I could get one that worked on cars...
No.2 ST1300 (one the you test rode for me) has a speedo healer, it was spot on too.
I don't mind putting slightly less k's on the ol' ST...
Reminds of a former work colleague, he was preparing an argument to Toyota re. the inaccurate odometer (on his then new in 1977 Corona) and the 12/12 warranty running out when the odometer reached 12,000km but the vehicle travelled less. Don't now remember the km involved, probably 500 or 600.
Might see him at a reunion lunch week, will check the outcome.