Not really sure what all the fuss is about this place. When we went it was absolutely packed to the rafters, complete with a 30 minute wait for a table.
The food wasn't bad - my bacon burger was decent, as was my wife's triple decker grilled cheese (with variety of cheeses). Both tasty (not amazing), without being worth anywhere close to the hefty price tag.
Where Serendipity 3 fell apart for me was in the atmosphere. The place is tiny, overcrowded and the wait staff are impatient, harried and DO NOT LISTEN. I tried to explain something specific that I wanted and our waitress seemed to be on complete autopilot as she tried to finish my thought for me without having any idea what I was talking about. Eventually I just gave up and hoped for the best (I didn't get what I had asked for). Wife's Grilled Cheese was supposed to come with soup and a choice of chips or fries. Waitress insisted she couldn't have both, despite the menu clearly listing soup as coming with the grilled cheese (most likely so they can justify their price for what is, after all, just a grilled cheese) by default and an "all burgers and sandwiches are served with chips or fries" note at the top of the section.
Perhaps we should have tried some of the things that people rave about here like the frozen hot chocolate, but our experience was one of mediocrity.