Extremely reliable. The 3.8l minivan engine in the earlier JKs, and the 2.4l 4cyl from the PT cruiser don't have a great reputation, but the 2.5l 4cyl and 4.0l 6cyl just keep going. I'm still driving mine daily, and I've got 224k miles on it. Transmissions and axles are reliable, but if you put giant tires and too much power in front of the transmission, things are going to break. There are volumes written about the best engine/transmission/axle combos work best for various types of off roading for Wranglers. In stock form, those components are perfectly fine.