My Explorer runs great overall. The main problem I have had has been with the Exhaust System. The exhaust manifolds are known to either leak (from the gasket) or crack. Typically, if you catch it soon enough, you can just replace the gasket with a thick copper or dead soft aluminum one and it elliviates the problems. It really doesn't cause damage, just an annoying noise. Most of my problems have been related to the bunk work my local dealer did when replacing the Exhaust Manifolds (under warranty) and is not the norm for most 5.0's.
Beyond that there really hasn't been a lot of big issues for the most part.
Fuel Economy on my X is good. When I first got it I probably got about 15-18 MPG depending on how much I was on it, but that was almost all city miles. Now that I have an aftermarket exhaust that's better flowing I get about 16-17 MPG in the city and have gotten up to 21 MPG on the freeway, but I'm a major leadfoot, so you could probably do better if you kept out of the gas.
I think the only real complaint I have is with the exhaust system. There really isn't much you can do with it because of how tight things are. Only Ford has made aftermarket headers for it and they didn't make a huge difference and aren't available anymore. Some people have had custom headers made, but that's some major $$$$.
The X is a very reliable car on a whole if you do all the regular maintenance and is also a great vehicle for towing with a Tow Capacity of around 7,000 lbs. straight from the factory.
Despite all my complaints its definately a reliable vehicle overall.