Big Turkey,
Determine what your use is then decide.
The Underdrive pulleys may be new to the Explorer set but they are an old idea as stated before.
The problem with the pulleys is that they do not drive the accessories at the specified rpm.
The alternator will produce less amps, the A/C compressor will not move the coolant the way it should, the water pump will not circulate water at the same rate to keep the engine cool.
Yes you gain a little HP but is the trade off worth it?
Consider an electric fan or a better air breather or maybe a set of headers with a good exhaust system instead of the pulleys.
Of course this is my opinion, but for off road use everything that matter is affected by underdriving the accessories and I for one do not want problems on the trail.
Good Luck on your chice,