It was pretty poorly paced, the integration to America was weak and awkward, and the solution didn't suit the previously well-established canon.
As I said: good, but by no means great.
Torchwood, for me, was the epitome of unfulfilled promise. It could have been the Angel to Dr. Who's Buffy, but instead came off like a teenager writing a sexy supernatural show. Children of Earth was awesome because it has real stakes and showed that the characters could really, truly, fuck up in astonishing ways.
Torchwood: Children of Earth = Amazing. Hands down, awesome.
Torchwood: Miracle Day = ... Hmmm. Good, but... not great.
As I said: good, but by no means great.