is this device accurate just like a driver? how does it differ compared to swinging a club at fullspeed?

Its not the world's most accurate gauge of swingspeed (for that you need other technical gizmos :D) but it does give an idea of your swingspeed. Remember the stick is heavier than a normal driver and its longer too.
Alex

it's close
 

I have a radar device that measure clubhead speed. The Speed Stik is really close. Drive distances seem to correlate pretty accurately with the measurements you get from the Stik.