I switch mine off - compliance with crew instructions is a federal requirement, as far as I know.
The instructions are, of course, rubbish. Constructing a cell phone with out an "off position" is sufficient to comply with instructions that state "any device with an off switch must be turned off at this time".
The instructions certainly clean up the plane's RF environment, but it's silly to expect compliance from even well-meaning humans at the 0.3% level (all of 400 passengers turn off all their devices).