Fun as HECK. I got into MUN my junior year of high school, wish I had gotten started sooner. Started in legal committee (Not my favorite) but senior year I attended the University of Michigan MUN and was placed as Prime Minister of Japan in a historical recreation of World War 2. We WERE supposed to try and work it out peacefully, but the US delegation wanted to brawl and had our Emperor assassinated within the first 3 hours of talks. We then proceeded to essentially play risk for 4 days. (10 hour days too) I won a gavel at that conference and I couldn't have asked for a better cabinet to have served with. My ministers of the Army, Navy and Interior were invaluable assets and we actually ended up completing a suicide run against the west coast that left San Francisco in ruin and most of the salad bowl on fire. We ended up losing because our sim-Hitler threw in the towel early to try and negotiate a better position for post-war Germany, allowing the Allies to devote all their resources to the Pacific. It was a brutal war too. As soon as bullets started flying I instituted terror tactics in the field, mass beheadings of POW's and the heads left on stakes. Whole islands covered with mines and not a single Japanese soldier to be found. Mega-doses of combat-meth/adrenaline issued to even the lowliest soldiers (Turns out the stuff is actually super cheap to produce, shocking), mass production of suicide torpedos, all kinds of fun stuff.
Your mileage may vary. But I will say this, caring about MUN makes you better at it. People who go in and half ass it, join committees they can't get fired up about, don't have fun and don't do well. That being said give it a go. Lots of fun to be had, lots of great friends to be made. Plus there is usually a delegate dance at some point (If you're into that sort of thing.) Also there's fun MUN related innuendo. (I'd like to table the delegate)