While they can't be dedicated transports, they are still useful.
I got a chanced to do a play test of the new codex at my LGS (huzzah for preview copies!) and was reasonably impressed.
They are definitely more of a gunship rather then a battle taxi especially if you pay the points to either run them as Vendettas (3 TL lascannons) or attach rocket pods and door gunners for much anti-infantry dakka.
I had a reasonable amount of success running a pair of them as a Veteran delivery team. They moved up, dropped two squads of vets; one led by Gunny Harker (Str 4 FNP with a relentless heavy bolter) and the other one by Lukas Bastonne (can give orders to his squad). Between the two of them they managed to neutralized two Ork Trukks and their squads.
That particular formation ended up running at around 660 pts.
Later on that game they managed to inflict a grand total of 21 hits on an ork mob (One of the valks was out of range with its rockets
In general, a valk heavy army will play very much like Mech Tau. Fast transports with hard hitting infantry back up while you have gunships in the rear providing fire support.