NLB,
But I have seen one used to good effect when the pilot actually called the tower from a PA 28 with radio failure and got on the ground with "proper" clearance even.
In fact this happened to us last week when on a local VFR flight from Shoreham in a PA28. We realised we had a radio failure when trying to call up Goodwood on the way to the IoW. We could receive but could not transmit. Cycled the radio and checked everything. Decided we would turn back to Shoreham and sqawked 7600.
Radio failure was confirmed when trying to call Shoreham and we received from them "Carrier wave but no transmission". We knew Shoreham was busy that day with a lot of circuit training. Whether or not it is illegal or recommended I switched on the mobile phone (I was in the right hand seat), flicked through Pooleys for Shoreham ATC and rang them. I was very surprised we could hear each other over the cockpit noise. Military had told them a plane was sqawking 7600 and heading for Shoreham. I gave Shoreham our call-sign and were told to join overhead blah blah as per normal, including info on traffic, runway, QFE, wind. When they saw us in the overhead they gave us joining instructions etc. We replied with a couple of clicks on the tx button. ATC continued to give us "normal" instructions on the circuit, cleared to land etc.
Using a mobile might not be "recognised" but it worked and the ATC were professional and very helpful. It is also very nice to know that squawking 7600 gets an immediate response!