Johnm is of course wrong, as there are plenty of VFR and IFR pilots quite happily navigating long distances safely without GPS.
However, he's also right that for enjoyable flying, a moving map GPS is far and away the best tool for primary navigation that currently exists, again both VFR and IFR.
But, despite this last, it's banned in the CPL skilltest, and not used by and large for the IR either.
So, if you are planning to go commercial, I think that the best best is to rely heavily upon DR / Plog navigation, and practice flying that way to the highest possible standard of accuracy.
But if you have a GPS, take the thing - used sensibly, it can keep you out of trouble, and provide a useful safety feature. "Nearest airfield" mode when the weather starts clagging in is a very good thing for example - you can beat yourself up about how you got into that position after landing.
I flew with a "student" who already holds a PPL at the weekend who was using an iPad based GPS nav system. Approx. 15 miles from his home airfield, the batteries went dead and he declared himself lost. Just don't ever let yourself get to that point. [And yes, even he agreed that some remedial instruction was required.]
G