Well i think there's a ton of opinions out there and it depends on so many factors (i.e. are you going to overclock, what kind of fans can you add into your case, what is the airflow etc etc). Probably best to google some opinions, but i think if you're not really going to stress your PC or delve into overclocking, air is probably sufficient (a good case can also help here).
My current case got a liquid CPU cooler and then air cooling for the rest. The CPU liquid cooler alone was like R1.4k vs. like R50 for a CPU fan/Heatsink ? So that is a huge price difference right there.
It is probably more difficult to install than a fan+heatsink , however these things come pre-assembled/sealed these days and hardly require maintenance . You literally buy it in a box and plug it into the case, assuming you have a case that can handle the radiator. To quote Cooler Master (the cooler i have)
http://www.coolermaster.com/cooling/nepton/nepton-140xl/
I think a fairly big factor is probably noise reduction , my PC is pretty darn quiet now, so the liquid cooling is certainly keeping things below a certain temperature more efficiently , my previous PCs would sound like a jet engine whenever i start to play heavy games as all the fans starts to go full speed...now it would still increase in speed, but less often and not constantly.
Either way, i guess it's tricky, i added liquid because i had some extra $ when i replaced my motherboard+cpu and figured what the heck, but i'm sure air cooling would have sufficed just fine, especially since my case (without all the DVD drive bays) got alot of space for big fans and airflow.