You can't go wrong with the slip on carbon Termignoni set up direct from Ducati. They are a bit pricey compared to some of the other brands available but they do come with a new race ECU and air filter set up. Best of all, there is no impact on warranty as mine were dealer installed at time of delivery on my new M1100S.
Depending on how you like your sound, these carbon slip ons have a removable db killer to make your Monster sound a bit more Monster. They are also available in stainless and titanium.
There are several posts in the accessories and mods section of this forum with all sorts of varied opinions. As with any customization to your bike, there is no right or wrong answer. It is what you like.