[rintinglen]

I have played them on occasion as part of an undivided army, but fundamentally, they are broken in fourth edition. 4th edition is all about shooting, World Eaters are all about charging and chopping. They aren't fast enough to get across the board without getting shot apart. If I play undivided then I use Raptors, a bunch of small squads built around heavy weapons, and an Obliterator squad. Then sometimes, I may livelong enough get a charge in, but the basic Berzerker is simply too expensive pointwise. He spends three turns getting accross the board, loses half or more of his squadmates, and then maybe kills a squad that costs half what his did. He gets one or at most two turns of attacks in. For the Same points you can put a 10 man Havoc Squad Armed with Lascannons and Missile Launchers on the table, and they can be shooting for the whole game if terrain allows it.
Unless there is a Land Raider, a big Terminator Squad, a Tooled up command Squad in a Marine Equivalent army, or perhaps a Monolith on the table, it is very Hard to assault something that is worth the cost of thesquad. And even if there is, all a smart player has to do is dangle a cheap bait squad (or vehicle) out there and the Berzerkers are off and running. Playing a standard mission is almost a joke if it involves taking objectives. Berzerkers are like Nurglings, they won't stay put and so cannot be trusted hold an objective.
Fortunately, Khorne cares not whose blood flows, only that it does flow.