I play a lot of Tau armor and have never found smart missle system to be that useful. Sure the no line of sight thing is cool, but it really doesnt come up that often, its the same strength as the burst cannons, but with 2 less shots, normally your not going to be near enough to use them either. Hammerhead Rail guns are cool only against armies with very few 4+ saves, or lots of really mobile armor.
I use Ionheads very frequently, mostly because they have AP 3. I use them inconjunction with my pathfinders. I dont normally use them as anti-light tank, they can do it. I use ion heads in 40k in flash to save points.
Ionhead: Necrons(!), Anything with Power Armor, Eldar (sometimes).
Railhead: Nids, Guard, Dark Eldar, Eldar, Orks, other Tau
Most of the time I use 3 broadsides (or only 1 if I dont have the points), one with a shield gen, and two with target locks. I dont use plasma rifles or multitracker. I use them as turrets primarly. That works out pretty well. It can be a little risky, because if you lose one they have to take morale test and they can only move and shoot. But there more accurate and can produce a greater volume of fire power than the hammerhead. Normally I place my 'heads in cover first, let my opponet play his armor (if any) then, place my Broadsides accordingly.
Armor Tactics (for beginners)
Place your 'head behind cover during set up, since it can move 12" and shoot (if you are not useing multitrackers stop playing Tau), your just as efficient moving as you are sitting still. You can move far enough so it should not hurt you.
ALWAYS move more than six, its a Skimmer, your opponet can only get glancing hits.
Skimmers can pop up can shoot over terrain, for some reasons nobody remebers this. Be careful though, you will probably lose a 'head doing this
markerlights IGNORE(!!) cover saves. Now those pesky lictors/jungle fighters/rangers are dont get there nice little benefit. Plus Ion cannons hitting on a 2+ is a good thing.
Fish of Fury is fun, but in doesnt always kill everything. Use it as a clean up tactic, unless your sure. Try to get the nose of your fish about 6 inches away from the target, that way the enemy cannot charge next turn (unless they can move more than 12")