A Warlock can be used to fire the weapon by acting as a crew member, this has been discussed before.
My personal experience of support weapons is confined to D Cannons, as they are the only ones I have ever used. They are great for area denial, so if you are playing a defensive game, using a castle tactic, or, even better, you a playing scenario whereby you are defending an objective or building, they are okay.
The problem is that they are so vulnerable to outflanking units, units which can Deep Strike, Fast Skimmers (e.g. Landspeeders, and infiltrators. As you can see that's quite a lengthy list of units which can take them out relatively easily.
This weakness is also true of the other support weapons.
As a result, I wouldn't suggest that you take them unless you are playing more of a gun line or static castle Eldar force, or you are taking a large fire base, since they will be too easily isolated and destroyed.