Quite reliable, relatively speaking. My wife's truck is an '03 Montero Sport Limited 2WD. We had it for several years before I discovered it was designed off of Mitsubishi's legendary L200 truck frame. Explained why all the suspension components seemed bigger than they needed to be. 6-lug wheels, heavy truck style wheel bearing adjusters, hybrid rear differential, etc. Everything seemed, and is overbuilt chassis-wise. The softer suspension of the Sport version may need to be addressed, rear springs pretty weak.
Hers has the 3.5L and it is a good engine, however it does have it's basic maintenance gremlins as mentioned above. Keep the cam seals, valve cover gaskets, up, which prevent oil loss, which leads to valve seals and lifters acting up. Also watch the coolant bypass system, overheating can sneak up too. Good news is the 3.5L block is solid, and once you catch up the often missed maintenance stuff, they are great vehicles. She won't think of buying anything else. We've replaced the leather interior, fixed/painted here and there. Due for a new set of tires soon.
I think they are the most vehicle for the least money, especially if you can handle most of the maintenance, or have a reliable local mechanic who can speak Mitsubishi fluently. Lots of folks give up on them when the valve seals fail and you see smoke at extended idle. Fix the valve seals, and usually you are good to go. Bottom end typically still strong.
Also called Challenger, Shogun, Pajero Sport, etc. around the world.
Mitsubishi Challenger
Older, but still relevant discussion includes Sports.
Montero vs Sport