Do you use it? If so - how, and in what lists? Mixes in units anyone?
I personally feel that Fusionhead offers some other ground force solution of MW lack, but 30cm is a bit harsh. maybe mix-in with regulars?

nope. i use 6 fusionheads in one. making quick attacks at 30cm with pop up and crossfiring recon formations and markerlights makes it ugly for reinforced armour formations.

2Dobbsy: what's your list besides fusion hammers?

Fusion heads definitely have a role in an aggressive list, but ion heads are pretty much useless.

I'm personally thinking about this:

hammers 6 300
hammers 6 300
hammers 6 300
stealth 250
8FW+ethereal+gun drone 325
hammers 6 300
crises+shaso 350
manta 675
hero 200

and using 2 of the 4 hammer formations as fusionhammers.

Ions are a lost cause, there really isn't any use for them that the rail or other units fill with the latter having other advantages.

why no one mentioned it before list was frozen, then?

It was mentioned often but no one came up with a good solution which also fits the fluff of the weapon.

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link, if you please?
with basing on tabletop 40k I can only think of making them cheaper, as ionhead is cheaper then hammerhead in 40k - but this can be problematic in tau situation(small fms) - maybe work it out making it "4 rail/fusionhammers or 6 ionhammers for 200. and 6/9 for 300"
without total basing I'd suggest 2x maybe. as what I felt looking at ionhead was mainly "more shots of slightly less potency"

Yep, the Ion issue has been brought up over and over again for years, but noone's come up with a decent solution. Honestly, they're just worth less points.

Ion heads perform great as long range AP platforms - something the tau now lack after losing the stingray. they're a helpful addition to Fire Warrior formations

I was very in favour of giving them +1 AP from their current stat, as I thought it would balance it.

But I never have any good ideas. :-)

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Fusionhead is fine as it gives you access to some range MW attacks and is a good close support tank, the problem with the Ionhead is not thats its completely rubbish - you wont instantly lose every game you play just becaise you take a formation of 4 Ionheads for 200 pts or add two as an upgrade to a formation for 100pts. Its just in the context of the list where you have so many great AP weapons available, the Railcannon version is just much more useful as a support option.