Lowtimer,
Helicopters tend to be flown with your head outside the cockpit most of the time; with such an unstable platform as the R22 chasing the instruments doesn't work, and you certainly need to look outside ALL the time when hovering or going into a confined area. Many of the R22s I've flown have only basic instruments - compass (but no DI), altimeter, ASI, VSI. I wouldn't worry if the ASI failed, because airspeed is not so crucial in a helicopter. I wouldn't care too much about the altimeter or VSI either, so long as I was outside controlled airspace. If the compass failed and I didn't know where I was, I'd land in a field near a road and then sort it out...or fly low and slow(ish) and follow the roadsigns. You see, we do have those as options.
However, one thing is absolutely crucial for helicopters, and that's rotor RPM. On most modern helicopters the engine (and therefore rotor) rpm is controlled for you by an electronic governor. If the governor fails, you have to adjust the throttle youself, by reference to the tachos, keeping the rotor rpm "in the green". Helicopters have a mechanical correlator to help you,; it's not usually that good, but so long as you've practised, governor failure should not be a problem. I've flown a few ungoverned helicopters now anyway, so I'm more used to manual rotor rpm control than I used to be. If the rpm tachos failed you have a low rotor rpm warning light and/or horn on most helicopters; you could adjust your rpm using audio cues, and know if it's too low by the warning light/horn. If you had no governor, no tachos, no light/horn...shouldn't happen (in the R22 all are on separate circuits to prevent it), but not a nice situation at all. You'd have to rely on audio cues, and how difficult it would be depends on your own experience AND the type of helicopter. The R22 has a very narrow green rotor rpm band, and would be quite hard; the Schweizer 300 series have a much wider range and you can hear it fairly easily. But in practice, to hell with that; if they really all failed I think I'd declare a Mayday, land in the nearest field, and sort out it all out on the ground.
Any other helicopter pilots care to comment?