I was in Prague last November. We actually picked this time because we thought it was going to be nice and quiet. But despite that fact that it was VERY cold (it snowed a couple of days after we left), it was VERY crowded.
We were told that Prague doesn't really have the typical "low" season right now as it's sort of the new "it" city to visit. There were a great number of Americans there but most were from neighbouring European countries.
Prague is hauntingly beautiful but we didn't enjoy ourselves as much as we should have. It was too touristy, too loud, too crowded. The beautiful Charles bridge that leads to Prague castle is lined with street vendors selling cheap and tacky momentos - it kills the feel of the city quite a bit.
I'd still recommend going there, but dont expect an authentic experience. I mean, I found myself sipping martinis and eating Sushi in Old Town Prague Square (which is all well and good, but I like to eat like a local when I travel).