Once was recommended to Erica and she decided to wanted to see it on a date night last week. I took a glance at Rotten Tomatoes and saw that it had gotten 97% on the critics tomatometer. I immediately closed the window -- if a movie is recommended a couple of times, I'd rather see it without any extra information that might color my experience of watching the movie. The next day the Regular Guy on XRT reviewed Once -- I managed to miss the review, but heard the DJs say, "wow, the Regular Guy really liked that movie". Great -- another data point in favor of the movie.
So, with the provisio that you enjoy patient (some might even say slow), intimate movies, I'll add my voice to those saying "go see Once".
If you are the sort who wants more detail before you see a movie, I'll note that this is a sort of a kind of musical -- but no one bursts into song for no reason -- the two main characters ("guy" and "girl") are both musicians and so they burst into song for reason. The guy is played by Glen Hansard of The Frames and the girl is Markéta Irglová, also a musician, and they really wrote all the songs they perform in the movie.
Oh, and I was a little distracted during the first part of the movie, because it was filmed with handheld (digital?) cameras and it looks a lot like a Neutrino Project (with better sound than we ever get).