I've found the Doom cruiser is quite effective, but not effective enough to justify its FP cost. I'd rather have an Aurora or Apogee, since they use less FP, cost less (I think), can take on a wider variety of ship types, are generally more survivable, and have more cargo space. As for the phase frigates, they're just too fragile and easily countered by most high- and mid-tech ships, while being less generally effective than other frigates for their FP cost. I'd rather bring a Tempest or Omen than either of them.
I feel the Doom needs an increase in armor and OP to justify its FP costs, or a reduction in FP to 16 or so (also, I don't really like Fast Missile Racks on the Doom, but that's probably personal preference). As for the frigates, they could use some combination of the following: additional armor, a medium energy slot (probably to replace one of their current slots), additional OP, or reduced phase upkeep.