Like most other, I find it too expencive when comparing to other units.
Regardless, I gave my Broadsides a try in my last game (last night). I fielded 3 Missilesides with SMS and Seeker Missiles.
Against Orc Horde army, they were brilliant. Their only flaw, post battle, was the lack of Fly (disengage and fire). They got locked down in second turn, and only fired Overwatch after that, but did not suffer a wound, not even a hit.
They did wipe Zagstruk, two units of Stormboyz and half a unit of Boyz.
The Seeker Missiles finished off a Truk with KFF hiding behind cover(-save). Moreover, they became the focal-point of my opponent, to the point where he ignored other units and ran for the Broadsides to lock them down.
I fielded mine with 6 shield drones. They did the job and prevented 2-3 assaults and saved many a wounds.
In my list, I also fielded Longstrike with two Ionheads. Knowing what I was facing, I would happily replace those with more Missilesides.
Also - I love how that unit looks.
But all that aside, unfortunately, they are too expencive, they lack Fly, they are slow - and in most cases they (seem) to force me to play old-style Gunline, or more defensively.
I love how T'au has become so much more versatile in 8th. And from what I can understand, Broadsides does'nt lend themselves very well to that.
Even so, Ill probably field all my Broadsides next game, just for the heck of it. They're so sexy.