I've managed to get from the aircraft doorway at Linate Airport to the centre of Milan within 30 minutes (I had to get the last train to Bologna or end up without a bed for the night - not good as a lone female ), but it usually takes at least 50 minutes to do this at a less frantic pace - 20 minutes to clear customs, 10 minutes to get a bus ticket, find bus etc, 20 minutes for the bus ride into town. Cost is peanuts! A couple of Euros.

To get back - you'll need to get to the airport at least 1 hour before, but I believe it's much better to give yourself at least half an hour of 'emergency' time. Why not arrive 1.5 hours before your flight to avoid stress?

Malpensa is a little larger to navigate than Linate. I'd say you needed 1.5 hours to get to and from the airport, and I would give just a little bit more time for the check in process - arrive at least 1 and a quarter hours before your plane departs. Again, give yourself an additional half an hour breathing space... just in case.

As to whether the trip is worth it... absolutely! Head for Il Duomo and explore from there. Word of warning if you intend to visit on a hot day, make sure you have a cardigan or shawl with you. You will not be allowed in to view the Duomo Cathedral if you aren't dressed in conservative/modest attire - no bare arms allowed --OOPS! Just realised - this won't affect you in November!!!