I can't speak for any other devices that may currently be on the market, but the one that you specifically mentioned (EZ-Jumper) was invented by a good friend of mine named
Don Crump, and it is a very helpful device. The EZ-Jumper facilitates much easier bridging for most jump shots, but is especially useful when you must either a) bridge very tall in order to jump over an obstructing ball that is very close (within 4" or less) to the cueball, or b) reach out across the table to get to the cueball. Even tall players with long arms and fingers are often frustrated when facing shots that would be jumpable if they were only more accessible, and the EZ-Jumper will give you access to many of these hard-to-get-to shots.
As far as the legality of the device is concerned, I suspect that you will find that it varies between different tournaments/leagues, which is not surprising when you consider that the legality of jump cues tends to vary as well. I actually spoke with Don today, however, and he told me that the BCA had recently ruled the EZ-Jumper legal for use in local events, but not in national-level events. The device just came on the market a few weeks ago, however, and they are still working to gain approval from other organizations.
If you visit the EZ-Jumper website,
http://ez-jumper.com, you will find some additional information about the device, as well as a short video showing it in use.
Aaron