i totally love the esp201. its very convenient to work with in eclipse/code blocks and can run C and C++. the documentation is a bit sparse though but thats mostly no problem as theres a great community behind it.
But the biggest point for me is the size + the price. Why would one use a Raspi or Arduino to read out some moisture-meter if it can be done with a 3 dollar hardware piece the size of a fingernail?