interesting question. what is your equivalent of the New Yorker? in the new york area my friends like this site:
ArtCal - The opinionated guide to New York art galleries
as for reviews, it really depends, i think that if there is a show you like, you might have to search for that particular show to see who has reviewed it. and for a lot of shows no one has reviewed them. it takes a lot of manpower, particularly in cities like New York and London.
i dont have like one super site or one prized place that keeps me informed. i find that most major publications at least LIST exhibitions, even if they don't write about them. so usually if i look in the back of the new yorker and the nytimes, it satisfies any art needs. word of mouth helps too for smaller, out of the way shows.