We couldn't find a decent hotel to stay in the night before our flight to Athens. We looked into hostels but 99% of them had terrible reviews and the cheap hotels were far out in the city with no access to public transportation. Milan isn't cheap when it comes to lodgings so we decided to suck it up and sleep at the airport to save money.
We had a few hours stopover in Milan after arriving by train from Florence. After visiting the Last Supper and Duomo, we returned to Centrale train station, where we took the Malpensa Shuttle (€5) to Malpensa Airport - some 30 miles away. The ride lasted about an hour due to traffic and we were dropped off at Terminal 2 (budget airlines). The bus then proceed to Terminal 1 (other airlines).
Malpensa Airport is MUCH smaller than you expect and it looks like everything is on the ground floor. There are hardly any restaurants or decent facilities so bringing food with you is a good idea. We got to the airport just before 9pm and it was completely empty. The cafés were shut down and janitors were already sweeping up the place.
There were a few other people that had intended to stay the night as well. Around 10pm, a huge gathering of Roma soccer fans came in for a night flight back to Rome - complete with police officers. I guess they take security seriously there.
I wasn't asked for my passport and there were no problems with the guards that patrol the airport. The lights go don't off at Malpensa (At Orly, they go off around midnight) so you definitely won't be getting some sleep unless you knock yourself out with pills. The chairs are uncomfortable but there are no armrests despite the lack of padding. There is limited seating at Malpensa so if you're planning to hunker down at the airport, get there a bit earlier to grab a seat.
I think based on my experiences, I wouldn't sleep at Malpensa again unless I had an extremely early flight. I wish I could have shelled out a bit more money for a nice hotel room but I'd feel guilty anyways. So if you're trying to decide whether or not to sleep in the airport, I'd stick with getting a hotel room. It's not worth losing an entire night's sleep and being crabby the next morning due to that fact.