the bar stoplock is useless (someone showed me at a garage once) but the one that covers the seering wheel is a lot better apparently.. 
There are different versions of the Stoplock bar. The one in the link posted by Arnold_Lane - the Stop lock Pro - is the sturdiest one, that resisted attack in the Auto Express test for 5+ minutes. The other two versions; Stoplock Original and Stoplock Pulsar both only resisted attack in the Auto Express test for 5 seconds.
There are lots of owners on fiestastoc.com that use a Disklok. The main criticism they have of the Disklok is storage when it’s not in use; it’s rather large (and a potential hazard/projectile in the event of an accident) to leave in the passenger compartment in a footwell or on an unoccupied seat, and a hassle to put it in it’s storage bag in the boot.
If your car is attractive to thieves, a good quality physical security device like the Stoplock Pro or a Disklok is likely to deter the opportunist joyrider who’ll probably move onto easier pickings, but it’s unlikely to stop the determined professional car thief / scum who steals to order.
