My players are loving it.
I've got a tiefling warlord lugging around a greataxe, specializing in moving his allies around the battlefield, a dwarf warrior swinging around a maul and specializing in braining people over the head, a halfling rogue loaded down with "just in case" weapons, but specializing in avoiding OAs and making sneak attacks with a dagger, and lastly an elf ranger who unleashes hell with her longbow, and has several powers to keep her at range.
Incidentally, it felt to me like she has preferred enemy still--whatever is closest to her is her preferred enemy.
What's been the most fun is watching them already start to flesh out their character as they advance into level 2. The fighter has started to specialize in improving his OAs, while the warlord is focusing on more healing through leadership. The ranger is just going for straight-up archery improvements, and the rogue has been focusing on making himself even harder to hit with OAs.
The fighter and rogue have been working together--the rogue provokes OAs from targets the fighter has marked. They'll most likely miss if they swing, and it earns them a viscious beatdown from the fighter if they take the attack. Several times now, I've declined to take an OA on the rogue because it just means the creature suffers a free hit from the fighter.
In the event the rogue can't work with the fighter (if he's badly surrounded or something) then the warlock, waving his greataxe around, slogs in and starts setting up flanks with his at-will, and even granting the rogue OAs if the target tries to escape the flank. It's brutal.
The ranger is actually pretty soloish--she just lets the other three tie up the front line, then puts arrows in anything she can. Ranged attackers first, then she helps with the main fight.