Running Case (Trimble) FX-750 on  275 Magnum.  Felt I couldn't justify the expense of RTX this year so switched to Omnistar for a 3 month subscription.  Big error!  All I have to do is drive near a tree and I've lost signal and takes up to 45 minutes to get it accurate enough to use.  Called Omnistar.  The response is basically "Yep, that happens" It was more polite and technical than that but the bottom line is unless you're someplace with no trees to go under and nothing above 40 degrees to the horizon to the south for a couple miles you will not be a happy customer