I've messed around with them a little bit, but compared to the other Heavy Support choices Eldar lists have, the platforms are the weakest in my opinion.
When you compare them to Falcons, you have no where near the survivability, nearly the same firepower (Pulse Laser + Star Cannon + Shuriken Cannon if you're feeling like it, can easily rival the destructive power of one to two blasts of a D Cannon), have a much better range and mobility.
When you compare them to Dark Reapers, the Reapers have MUCH more shots, more accurate balistic Skill, and even have a better save, and while not as powerful, have really good AP, and twice the range of a D-Cannon. (And when someone says that Reapers have a higher survivability...wel l thats saying something....)
When you compare it to a Wraithlord, you have a MUCH sturdier, mobile fire base to put a weapon on, that can hold it's own in close combat, and can't be killed off by a random squad of guys with Bolters...
When compared to a Fire Prism, you have a much farther range to work with, and a much more mobile AND survivable fire base.
When you compare it to the platforms that join Guardian Defender squads, the crew can at least hide in the squad, meaning that it takes concentrated fire to take the pesky things out, with only two crew on the Heavy platform, and maybe a Warlock, you aren't looking at much, also, if you brought a single platform, killing a single troop now requires a leadership test...
In your choice between a Falcon and a Platform, I would probably suggest the Falcon, as you can also hide a small squad of Fire Dragons or Storm Guardians (With Flamers, & Destructor for mass troop hunting, or Haywires, Fusion Guns and a Singing Spear for tanking busting) in it (Depending on flavor preference) to fill small holes in your army and do some battle line patching.
- Newton