Good. If we had a proper 5-point scale, it would've been Above average, but not quite fantastic.
All in all, an enjoyable look at the the Doctor's character and an interesting look at a potential Mars colony. The colony was well portrayed and you truly felt stranded out there with them. So, lots of pluses there.
Downsides:
Overall, the characters were not really memorable.
We had to be told how great the colony leader Adelaide was rather than shown. She seemed just like many other leader figures in DW but yet she was supposedly somehow special? I don't buy that she personally (even if through her death) would've been responsible for humans colonizing space. We've got that drive as a species. If she had lived and hadn't become such a defining figure, someone else would have. So, the whole thing about her having to die didn't work for me. Didn't make sense.
I also didn't buy into the Doctor's character becoming so maniacal at the end. RTD was clearly going for an emotional rollercoaster at the end. Euphoria at surving followed by devastation, and then desparation. Fell flat. The Doctor, after a thousand or so years of living, isn't suddenly going to go mad like that.
The progression is logical until the end. The Doctor arrives, initially wants to leave upon learning where/when he was, being drawn in, and then refusing to follow the rules in order to save them. But, he wouldn't be so wanton, so callous about it after having saved them.
So, in summary, great ride, interesting setting but didn't buy the premise of how important Adelaide was nor the Doctor suddenly going mad and power hungry. Enjoyable but with a few faults that kept it from being great.
Mr Awe