I've used a Manta 3 time under the present stats (6.2).
In the last game, it was engaged at FF range by Marines. It (easily) lost the engagement and was broken. Criticals were rolled and the Manta
lost it's Deflector. It died VERY quickly after that.
From my perspective it is underpowered.
FF5+ is silly on this unit. It should be better.
Lack of ML is wrong.
A non-Fearless Titan is really easy to kill.
It is a combat transport. It's point cost includes the transport ability. At some point in a game, it has to close with the enemy (to deliver it's cargo) at which point it's easily assaulted and broken. Once broken, it is dead.
Opponents of the Tau are easily overawed when they see a Manta on the table.
In my first game using one, my opponent just ignored it. I won.
In my second game, my opponent took a couple of pot shots at it but nothing serious. I lost.
In my third game, my opponent went after it with powerful assault based detachments. The Manta was dead by the end of the 2nd turn, along with all it's cargo (that was 1700pts wiped out or broken in one engagment for the loss of 1 Marine stand and 1 LandRaider Crusader... ugly And yes the cargo did get out and fight, it made no difference as the Tau FF ability is artificially nerfed to shape how the Tau play). Needless to say, I lost ÂÂ

.
I would've thought that a Necron force, with all it's good quality FF would be able to make short work of a Manta.
I've found that the Tau work better with more feet on the ground (so to speak). I'm including an Orca filled with Crisis Suits to hit anywhere on the board (once enemy AA is dealt with) but I've found that if I steer clear of Manta's, AX-1-0's, Barracuda's etc, I have more success (at 3000pts).
If all the concerns were addressed and the Manta improved, it would still be worth 700pts. At present it is overcosted.