This weapon is interesting. At first glance, I looked at it and went "Oh, so it's still worse than the Pulse Driver...", but having thought about it more this morning I'm not sure that's true.
Some pros of the POMD:
Has built in ignore cover on the S8 AP3 version
Can D weapon lots of stuff on the 7" D weapon one
Those are two pretty strong arguments for it. On the other hand, the Heavy Rail Cannon, as shasocastris mentioned, is much more specialized. It is both anti-GMC\SHV and anti-infantry. The POMD does anti-infantry better, but if you want to hurt the infantry, you're sacrificing your D weapon. (Stating the obvious, but worth stating). HRC lets you keep damage up on GMC\SHVs that Tau normally have a hard time dealing with (anyone else find it hard to hurt a Wraithknight?) and still suppressing lots of other units.
The way the math works out, 11/36 chance of the "pick your model up", 24/36 standard D3, and 1/36 nothing happens. That is almost the same as "add +1 to the D table result." Just for some perspective on how good the weapon is.
I'm liking it comboed with the fusion arms. That thing is just a hard target monster, that doesn't die. Use the S6 AP4 to pepper things the same way you would with the SMS & burst cannons. It gives it an astounding 8 different guns to fire.
The biggest downside I see to it is the fact that it's a blast weapon, so it can't target invisible things, which means invis WKs are still a hard target to deal with.