I generally don't recommend them except in 2 cases:
1) You want the absolute max possible cooling, regardless of value and cost. There are no $300 air coolers because after a certain point, you just can't hang a larger piece of metal from your motherboard. A liquid cooler allows for a larger cooling surface than you could get with an air cooler.
2) You don't have room for a large air cooler, but you still want to heavily overclock your CPU. Maybe you have a small form factor case, or have some other reason why you don't have the clearance for an air cooler.
Liquid coolers don't offer a ton of value, in terms of how much cooling you get for what you spend. A $30 tower cooler generally matches or beats $80 watercoolers, and an $80 tower cooler generally matches or beats $150 watercoolers. If you're not pushing your CPU hard, that $$$ can be better spent elsewhere. AiO's are also generally more noisy than air coolers, and have moving parts which can and do fail (pump), so if you're looking for quiet or reliable, air coolers can do it better.