GPS is mandatory not an option !
If you fly above FL100 it is mandatory to have a BRNAV unit fitted to the aircraft.
Most of these units us GPS I have flown three types of airliner for UK based companys that have used GPS and only GPS to meet the BRNAV requirement, Yes the aircraft did have a full "airways" avionic fit but if you think that anything other than the GPS was used when cleared to an airway intersection 500 miles away then it is time for you to wake up and smell the coffee.
A GPS approach has recently been published at Lille, thats not very far from office of the luddites of Gatwick. How long is it going to take for these people to wake up to the fact that the rest of aviation is leaving them in the dark ages ?
The fact is that no one uses ONE and ONLY ONE type of nav aid , you always back up one aid with another and these days most of my IFR flight uses GPS with VOR/DME back up.
The only warning I would add to the above post is the GPS unit must conform to a TSO and be installed in accordance with that TSO, also it must have an up to date data base. The hand held GPS units are very clever but not up to IFR flight.