they're not very good, they die too fast, and suffer the meltagun curse
(if its an important shot, that you've committed yourself to cos of the range, then its going to miss)
basically they have a fair bit of potential threat of what they can do, a fair amount of options.
but each time you use them to do something, or even bring it out of hiding, then you need to assume it will die immediately.
so it can do a lot of things (not particularly well though) but if you try and use it to do something, then consider it a 1 shot weapon and that it will die. so you better make sure you use it for something worthwhile.
i use one in my farsight army, because otherwise i have no fast objective claiming. and, farsight suffers hard against indirect ordnance so i need the fast fusion to try and get rid of it if i can't kill it directly.
in a normal tau army i wouldn't bother.
oh and they're definitely only worth bothering with if they have fusion, and the TA is important too, making them 70 pts for the standard one i think.
finally, i wouldn't ever want to use them in squads, use them seperately if at all possible