Is it a masterpiece? Probably not, but who cares. I didn't really agree with the reviews about it being boring, or plotless. The radio series and the books were pretty feckless and provisional -- I imagine that when they were doing the radio show they weren't thinking any farther ahead plotwise than the next episode. Which wouldn't justify a bad movie, but this is a good movie. I think reviewers who weren't impressed probably didn't get the novelties the movie brings to the table, which are mostly a matter of tone and attitude. The book and radio series were pretty cartoonish, and you have to put more effort in a movie into making the events, no matter how absurd, actually appear to be affecting real people. The actors underplay their parts, and some of the best lines are thrown away. Martin Freeman especially nails the tone, which is a little darker than people might expect.
And the whole cast is pretty strong. Zooey Deschanel doesn't have near enough to do, but she's really appealing when she's around. Bill Nighy is, as always, amazing -- if you've never seen Still Crazy it's worth watching just for him. Alan Rickman again cements his position as the actor most likely to win an Oscar in the category of petulant bitchiness.
In thinking about it the whole movie feels pretty rushed compared to the radio series, which after all, ran several hours in 1/2 hour installments. I think there's always so much lost when something big and expansive gets compressed to fit in 2 hours, but all in all I think that they did a worthy job.