I only made it as far as about episode 4 of the first series of Life on Mars, then got bored with the whole idea. I tuned in to Ashes to Ashes because the one thing I had enjoyed was Gene Hunt. I'm still watching, although my attention did wander in the episode before the last one, which was a bit 'preachy'. I'm finding the Alex Drake character very enjoyable, far more so than the original bloke, who I tired of. Gene Hunt has been a bit downplayed in this series so far (but then he is supposed to be ten years older) but he came into the last episode quite strongly.
So far as the 'secret' of the show is concerned, I've got my own theory, but there was a great little exchange at the end of the last episode:
Gene: So, I'm just here as a figment of your imagination?
Alex: Mmm. Yes
Gene: That'll do for me Bolly
(Alex's nickname has been derived from Bollinger champagne and there is simmering chemistry between them)