Technically the word 微妙 means
as per WWWJDIC:
(adj-na,n) (1) delicate; subtle; sensitive; (2) difficult; delicate (situation); complicated; (3) doubtful; questionable; dicey;
However, last time I stayed in Tokyo ( about a year ago) most of my Japanese friends (about 25-30 years old) were using 微妙 to mean "kinda sucky" or "boring in an annoying way". I believe I remember a sentence like this:
How was the beach? Eh... it kinda sucked.
I also heard it used as an adverb with this meaning but I can't remember an example. When I asked my friends about it, they said they don't really use that word for positive things, though none of the numerous examples I saw on ALC had this slangy type of meaning. I guess the meaning of subtle mixed with dicey and then became "so either-or that it just sucks"? If anyone can explain the subtleties of this slang usage, I've been wondering for a while.