Easiest way to put it is, are you one of those people who play Grand Theft Auto games, but never finish? Saint's Row is basically a sillier version of Grand Theft, it really embraces the whole fantasy of being a crook. In GTA games you steal cars. In Saints Row 2, you hijack feces throwing waste trucks, jets & helicopters, and you can rock mini bikes & flying saucers if you have the special content. Side missions include helping film a 'COPS' like tv show. Saints Row 2 was more exceptional, because there are some great story moments that kind of shock you ( referring to the events in the Brotherhood storyline ), because of all the previous silliness.
I consider it one of my all time favorite games. If you're looking for reference as to what my favs are: Uncharted 2, Batman: AA, Halo, Call of Duty MW, Gears 1 & 3, Marvel Ult Alliance, for examples. Saints Row 2 is a great open world game that offers quite a bit of replay and side missions. It also features co op. I heartily recommend the game!!
Word of advice though: If you do a mission with a friend & you're trying to stay low profile until you get to the mission. Don't let your friend get a chainsaw & see a group of hookers. It NEVER ends well!