I've used my Garmin 196 in a R22 Mariner, and it seems pretty accurate. The GPS and built in HSI is really helpful when offshore and you don't have any land reference to fly towards (magnetic compass in the Robbie SUCKS). Anyway, the Robbie has small blades and the bubble is pretty thin, so reception is quite good.
When closer to home, I've cross-referenced GPS position to small islands on the map; it is dead on target (even without getting WAAS signals). Robinson has external GPS antennae mounts for their panel mounted Garmin 430's (mounted on tail section). I wonder if it makes reception that much better? (I've been putting my 196 on top of the instrument panel with Velcro).