The first thing to remember with AIO liquid coolers is that performance depends entirely on ambient temperature. Whereas an air cooler exhauts air away from the CPU, AIO coolers do the opposite. The air passes over the radiator which cools the liquid inside, so if the ambient temperature is hot, the liquid temperature is also hot.
A top-of-the-range air cooler (Noctua NH-D15) will certainly match, and usually surpass, entry and mid-level AIO coolers (Hydro H80/H90) and both cost about the same. In its favour, an AIO cooler is able to bring high temperatures back down again much quicker than an air cooler and they do look considerably better.
As said, AIO coolers aren't particularly quiet. Yes, the fans can be replaced, but the pump can't. The pump is also another potential point of failure, and whilst unlikely, there's also the possibility that an AIO cooler will leak.
I completely agree with Gam3r01; 120mm and 140mm AIO coolers aren't worth it and an equivalently-priced air cooler will do a better job with less risk. An AIO is only worthwhile if the radiators are 240mm or 360mm and the CPU has a large overclock.