They have improved in cost and ease of use. I keep looking at them but don't see the utility of adding one. Many models and manufacturers to choose from now. I even saw some sitting on the shelf at Frye's a while back.
Most don't cool any better than a good HSF does at a third the price. You need an expensive energy guzzling vapor system to get uber cooling if overclocking is what you want to do. Otherwise quieting the system would be the reason to go liquid.
Even with the improvements I'm not sure liquid is any more worth the effort than they were before though.
-CPU heat is coming down rapidly with Intel and AMD CPUs. 45-65W on new models vs 95-120W on older models.
-Video card heat is coming down too, though not as much as the CPU's.
-Most chipsets can be run with passive coolers.
-With power demand in CPUs and GPUs down that means the thermally controlled power supplies will be running fans slower and quieter.
-The 120 mm case fans are pretty quiet.
So quieting your system isn't as much an issue as it was before.
Then there's the risk of loss of coolant or damaging the video card putting on the video cooler blocks. Most of this has been addressed now. They use special coolant liquids that won't trash your system components if they leak. They monitor the coolant temperature (but not flow rate). Installation is still tricky and you really need to pick a liquid friendly case design.
HD and Optical drive cooling is getting more important so case fans are still needed.
Just doesn't seem worth the effort uness your trying to way overclock. But why do that when performance of current CPUs and GPUs is so high?
I prefer to put that $100-200 into better components to start and skip the hassles.
A fun project though if your bored and sitting on an unused tax refund.