I find the berserker to be among, if not the strongest Shock Troop infantry classes. Especially in Captain mode they can tear through other units quite well. But they should be used as support for Varyag or Warriors. Even in skirmish they are very good, but requires you to be able to dodge arrows, or stay in cover until people close in. Their primary weakness is their lack of defense against archers and cavalry, but with the throwing axe, they can sort of deal with cavalry.
As with all shock infantry, they do rely on team work to be effective, especially in Captain. In Skirmish they are good flankers.