Missions have been all the craze lately since they offer great loot and are available 24/7. Supposedly you can participate in 10 at once AND run 10 at once. I actually have yet to find more than 1 Mission (of my own) at a time and have probably maxed out in helping in 4 at a time.
Some food for thought on Missions:
Mission Loot: While the Easy level Missions may not drop loot that makes your top 501 (usually the stats are in the low-mid 40s), help out a Mafia member who finds an Easy Mission - we're all in this together.
The Hard level Missions are dropping insane loot - the stats make Las Vegas a memory and blow away anything in the Armory, Weapons Depot, or Chop Shop, in pure numbers. Lots of nice Animals as well, especially in the Rare or Uncommon categories:
That Sleeper Shot with its 73 defense... good stuff.
The Common category loot from Hard level Missions will still rival most of what you're getting in Las Vegas... except Vegas now has a Loot drop frequency of about 1-in-30 jobs on the higher Districts...
The Death Dealer Minigun (48/64) is one of the best stat-Weapons around and will put your Titanium Katars (50 defense) from Bangkok - and even the Range Finder Rifles (54 defense) and Pitbull Sentry Guns (52 defense) from Las Vegas to shame.
The Medium level Missions drop loot on par with Las Vegas District 6 or so - definitely worth getting - the Weapons and Vehicles are like extra Railguns and Hunters...
...and even better if you collect Rare or Uncommon loot on a Medium level Mission.
Mission Manners: Nothing is worse than signing up for a mission, or recruiting for one, and watching 2 days pass without it being completed. If you recruit for a mission and can't get all 3 or 4 slots filled quickly, call for help again every couple hours.
If you sign up for a mission and don't have the energy or stamina right away to finish, make finishing a priority. There is no excuse for nearly 2 days going by without some one putting in the 100-120 energy or stamina it takes to complete his or her part.
My proposed punishment for those Mafia members who join - but don't complete - missions in a reasonable amount of time is to exclude them from the Mission (and hurriedly search for a replacement), and consider removing that person from your Mafia altogether....
Mission Timing: You've got to be a speed demon to participate sometimes. Once you see this symbol on your Facebook news feed...
...you've got about 28 seconds before the three or four crew members have filled the job. It's insane how fast these missions are responded to... but the wealth of good loot available makes them worth trolling the boards and clicking quickly.