I have no experience with other meta-build systems, except QMake. So I can't really compare.
In my opinion, what makes CMake a good choice is its power and flexibility. It's very complete, yet it allows you to tweak almost everything -- and the bigger your project is, the more you need to customize the build process.
The downside is indeed the complexity of learning it. If you look at SFML's CMake files, you'll find many tricks and commands that required a few hours of searching in the doc and testing.
The support for various OSes / compilers / build systems is very good, except for Mac OS X frameworks apparently.
CTest (unit tests) and CPack (packaging) are also very convenient if you need these features, they integrate very well to the CMake files.
So my conclusion is: if you're motivated enough to learn it, it's worth the effort.
If you have more specific question, just ask