I watched the US Life on Mars because I loved the UK version but thought it was entirely possible to do something very different but good with the basic premise.
Overall, I liked a lot of episodes of the US version, but it wasn't what it could have been. First, the casting was bizarre. The UK Sam Tyler is a relatively smallish guy who can be physically intimidated by some of the 1970s cops who are bigger and rougher, Gene Hunt not least. The US Sam Tyler is the biggest guy in the police house, and Hunt is a small old man. Completely different dynamic. There's also the odd lurching feel of the US series, as it experiments with some ideas and then drops them, feeling like it's making up a lot as it goes along, and goes through some drastic shifts in tone and style. (To say nothing of the original pilot, set in LA instead of NY. Even Colm Meany as Gene Hunt couldn't save it from utter worthlessness.) To be fair, they started signaling the series ending pretty early on. But it isn't nearly as consistent as the UK version.
Ashes to Ashes... the first series never really worked for me until the final episode, but I thought the second had a lot of good moments. Looking forward to the third.