Writing non-trivial functionality in cmake's language is possible but you're fighting the language the entire way. It would have been so much better if they'd just started with something like Lua and extended it instead of building their own barely-adequate scripting language. Some of the grossest code I've ever written is in cmake macros.
On the other hand, the results you get from cmake are fantastic. Want to use ninja instead of make? Just pass a compile flag. Need Visual Studio project files? No problem. Xcode? Kdevelop? You're covered.
Honestly I couldn't imagine ever going back to autotools at this point.
Having worked with CMake, it seems more expressive than Make (meaning you don't have to contort it to make it do what you want), but it seems quite complex I the sense that I don't have a clear picture of its semantics, i.e. what's going on behind the hood.
cmake is a replacement for the autotools suite, not make itself. You're right that it can be tricky to use.
One advantage cmake has is that you can tell it to emit files for ninja instead of traditional Makefiles. ninja is very well optimized for the problem of parallel building.
