i have one of the cheaper variations on the bob, i got it for £100 after a bit of bartering and i bought a rowing machine aswell
as for actual usage id say its very helpful for improving things like accuracy of punches and kicks, i also find it helpful to be able to try techniques i have learnt in class on it to see how it relates to a real(ish) person without fear of injuring a training partner etc.
however it can be misleading to think it is the complete training tool as it does not react to your attacks in the same way an opponent would so always be very aware of things like your guard and speed of techniques, it can be very tempting just to smack the hell out of it and then develop bad sparing and real life habbits as a result
overall though i would say mine was money well spent, but £250 for the proper bob seems alot as i dont think it will be that much better than the other free-standing bags on the market such as this:
http://www.simplyfitnessequipment.co.uk/Free-Standing-Bags/Torso-Man-with-Flexible-Post-Base
hope that was of some use