My experience has been that the tool works 100% of the time. It's a cheap little piece of sheet metal, but aside from that I don't know what problem people have with it. You only need it to grip the chainring nut flats for an instant when you break the bolt loose. THen you can generally hold the nut still with one finger while you finish unscrewing.

THE SOLUTION to difficult chainring bolt removals is selective lubrication - first, wipe off any oils or greases or contamination from the outside of the nut (where the nut meets the chainring or cranks), then add a small amount of grease to the threads of the chainring bolts, but on no other surface. IF only the threads are lubricated, and the surface of the chainring nut that mates with the chainring is dry, the friction of the nut against the chainring will be much higher than the friction of the bolt threads against the nut threads and the nut will generally hold itself in place when you tighten or loosen the bolt.

Another option for when you get the bolts off is to find a set of chainring bolts with a nut that accepts a hex head instead of the split screwdriver tool. The strategy of differential lubrication is still effective, but it's much more likely you'll have an allen key handy than the chainring spanner when you are working on your bike.