Honesty, I just don't feel the WWP is worth it in 6th.
For a start, it's a costly investment - 1 WWP is risky (especially if you don't get first turn), and 2 will cost you 200pts minimum (including the haemonculi). And, sine those two haemonculi have to disembark to drop the portal, they're unlikely to see another day. If one of those is your warlord (since not every list can afford 250pts of HQs), then your opponent can get Slay the Warlord, and possibly even First Blood in his next turn.
Now, in 5th, it could lead to a potentially high-reward strategy. After all, once they were down, we could move Beastmasters, Grotesques, hellions, wyches and even Pain Engines through and charge in the same turn. Considering the fragility of most of those, this was a considerable advantage. And, if nothing else, we could at least make the centre of the board a death-trap for enemy units, as long as we still had reserves (which could help force your opponent from objectives etc.).
However, when you can't charge the turn you arrive, the portal seems a lot less threatening. Before, it was a death-trap for our opponents - now it's a death trap for our own units. Once they emerge, they now have to sit through whatever rapid-fire, large blasts and flamers our opponents have handy. And, in the likely event that our reserves arrive at different times, our opponent can simply focus on eliminating a few units at a time, taking our army apart piecemeal.
I know there's the possibility of using the portal for shooting, but it seems like we don't have enough durable, mid-range stuff for that to be viable.
If we do get a 6th edition codex, I'll be interested to see what happens to the WWP.