Barrier is what I'd consider an 'ace in the hole' ability. It can just do SO many things. The most obvious use of this ability is to put up a wall between yourself and your enemy. But here is a list of other things I've done/had done to me that were very cool/effective:
Putting a barrier of protection around one particular figure that you've decided to put out in the open (read: Nightcrawler). Most forces will just leave him alone, but those that don't still have to waste a 3 dmg attack taking down at least one barrier, meaning that they have used on action and that is 3 dmg not coming at one of your figs.
Putting up the old wall to separate an enemy bruser from his taxi...
Blocking off LOS/adjacency for things like: Defend, Probability control, SUPPORT, etc.
Surrounding the enemy 'big hitter'. That big hitter probably costs more points than your two barrier figs put together (and quite possibly, then doubled). One action per turn is all it takes to serously hamper an opponent's force.
The one thing to remember on some of these tactics is that the opponents must be adjacent to each other on diagonals. I've found that most opponents don't make a specific effort to keep them in direct path, so many times these things work. The key here is to look for the opportunities to exploit, as most of the time, opponents are expecting a wall between, not some other (often more effective, if only because he wasn't expecting it) manouver.