I have about 40 hours in Global Agenda and levelled a few characters, but I can't see myself paying for anything. None of the classes interest me enough to play one exclusively and it's really only an XP/loot boost and the use of the auction house so I can't be bothered.
At least it doesn't artificially limit your playing time like Spiral Knights does with "energy". I can only get an hours worth of gaming out of it at one sitting. With energy being so expensive, there's really no incentive to come back (for me anyway).
tl;dr: No, but maybe if the game is right.