I laugh all the time!
I don't know about a giggle fit, but I laugh a lot in class, and almost always have a smile on my face.
I laugh when I get thrown in some unexpected way by my sensei, or when the connection is *so* good that I seem to go from attacking to flying without a transition. I laugh when I'm learning a new and wonderful technique -- something that I hadn't seen before or that's surprising in its effectiveness and simplicity. I also laugh when I make silly mistakes. These aren't really belly laughs, but pleasant, soft laughs.
That's not to say that all of my training is a completely joyous experience, but enough of it is to keep me coming back for more. I recently attended the Aikido Association of America Summer Camp 2000 and took classes from Fumio Toyoda Shihan (6th dan) and Yasuo Kobayashi Shihan (8th dan). Both men are wonderful teachers, and both approached Aikido with a sense of humor. Kobayashi sensei would often make jokes, in a kind-hearted manner, at his uke's expense (including me), and Toyoda sensei was always making small jokes. These men taught me that one can train seriously in Aikido while still training joyfully.
Our time on the mat is special. Even though we are studying -- at least in part -- to prepare ourselves to be able to handle a violent encounter, that doesn't mean that our training can't be an enjoyable experience.