I absolutely love the devastator squad models. I love the look. They are Marines with huge weapons, ready to blast things.
You can see my devastator squad in the picture getting some action in an apocalypse game that paired my Blood Angels with the Black Templars of a fellow player in our group.
I found out after I had already finished modeling and painting that I had paired anti-tank and anti-infantry together. I really should have put Missile Launchers in with the Lascannons.
Regardless, the devastator squad is too expensive to use in a 1700 or 1800 point tournament list. Just the five marines kitted out as I have them here costs slightly less than a Land Raider. I guarantee you that they don't survive as well as my Land Raider. So, I don't use them in my tournament list.
That doesn't mean that you can't use them. If you are going to use them, I feel that you have to increase the squad size to 10 marines. Why? Because they will attract attention. They pack too much punch to be ignored. Your opponent will try to kill them. You need those 5 extra marines as wounds, cannon fodder. The squad has to be able to soak up some extra wounds before you are required to take one of those expensive Heavy Weapons off the table.
Suffer not the traitor to live! Death to Xenos and Heretics in the name of the Emperor.