You are interfering with the ability of others to communicate, it is the same as gagging everybody around you, which is about as rude as you could possibly get.

When somebody wonders out loud why their cell doesn't work, especially if the call they need to make is an urgent one and you whip out your 'gadget' be prepared for nasty stuff to happen to you.

Interfering with any kind of communication is about as low as you can go in my book (other than doing violence to other people), doing it on the sly makes it one step worse.

Talking loudly on a cell phone, oblivious to the tranquility you destroy around you -- that will come pretty close.

I'd like to think restaurant patrons and mass transit commuters had consideration for others (and the presence of mind to talk quietly) but they often don't. Jamming is probably not a good solution, though.

Society is about playing nice. To talk loudly on a cell phone is not nice, jamming isn't either (and illegal besides).