There aren't many. You get to go to a special line when you first check in. For us, the pier was very crowded and all the lines had a wait. You don't get into the latitudes line until quite a ways in (almost on board) and we waited a good hour before we even got to that point. One woman was highly insulted that, as a latitudes member, she had to wait. She wanted to cut everyone and we told her that hey, we were latitudes members as well, but she was still looking for a way to get around everyone. We maybe saved 5 minutes by being in the latitudes line. I'm sure on other days you might save considerably more time, given the amount of people waiting.
Also, there is a latitudes party that is strictly for members. We went and there were free drinks (champagne and a few other things) and hors deuvres. It lasted about an hour and wasn't really a party. You just kind of hang around and they talk for a minute (thank you for being a repeat customer) and then they have a raffle. We didn't win anything but they gave away a bottle of champagne, some t-shirts, and a beach towel.
There is also the discount you can get for booking your next cruise. They say you can save more by booking during your cruise and then being a latitudes member will also get you a bigger savings. I can't say much about that since I didn't go to any of those meetings but I'm willing to bet you can do better by booking online.
There were no other benefits that I am aware of. Maybe someone else will know of more. I sort of expected there to be a fruit basket or something in our room when we checked in but there was nothing.