Days was absoloutly awsome, probably one of Danny Boyle's best (next to trainspotting)- it felt like a 'proper' film, it is well paced, belieavable, new what its themes were and executed them well. It was the kind of a film a person who hated zombie films/ horror films could have watched and really enjoyed.
Months was ok. The first 10 minutes with Robert Carlyle escaping the house were brilliant, but the rest of the film fails to live up to it. It's pretty contrived in places, generally it feels like they had a bunch of action sequences and threw in a plot at the last minute to hold it all together. Saying that, it is well shot, there are some good scenes in it, and makes good use of the rather brilliant "in a heartbeat" theme music. It's like the godfather III, worth watching if you liked what came before, but if you never saw it you wouldn't be regret it too much.
Interestingly Danny Boyle was involved in the sequel, I think he helped produce it or something.
Last I heard they were making a 28 months later and it was being directed by Danny Boyle. Could be good, could be bad. I'm more interested in Ridley Scott directing Alien 5.