They're easy to use, and adhere better than pastel powder for instance. Easily cleaned up and convincing appearance. I think I read somewhere that they can be sealed with an overcoat, e.g. Future, without as much pigment loss that you get with pastels...like with pastels just don't spray or brush it on real heavy. But then, they don't come off as easily as unsealed pastels anyway, which can come off very easily just from handling the model.
Some people report that the included applicator isn't particularly sturdy and wears out fast. That's when you make a trip to the beauty aisle in Ye Olde Grocery Store and buy some of those disposable makeup applicators to replace it.
Most people's comments I've read have been positive for the Tamiya kits, but I'm sure there are some who don't like them for one reason or another.