Yesterday I went against my personal rule of not paying for someone else's advertisements and purchased the latest White Dwarf with the new Spearhead rules. The expansion allows you to field a certain number of additional vehicle, walker and monstrous creature units to a normal Warhammer 40,000 army. You purchase your army using the normal Force Org Chart and add on a Spearhead formation that is outside of that. Armies of the Tau Empire will be able to fill up Heavy Support slots with Broadsides for some nasty anti-tank firepower while still fielding a nice detachment of Hammerheads with Railguns. Piranhas, too, can be fielded as part of some Spearhead Formations and I can see their uses in the Skyfall Formation, deep-striking in to roast enemy tanks with Fusion Blasters.
All Spearhead Formations are piloted by elite tank crews and no longer need the Multitracker upgrade to fire one weapon whilst moving at Cruising Speed, as all Spearhead Formations can now do so. However, purchasing a Multitracker will allow you to fire both your Railgun and your support weapon, be it a Smart Missile System or one of the Burst Cannons. Although I'd suggest just saving the points on the Multitracker as you won't need the S5 weapons.
There are two formations that stuck out to me as potentially abusive with Tau Hammerheads; Ambush Spearhead and Tank Hunter Spearhead.
Ambush Spearhead gives your units the Stealth and Infiltrate special rules although you can't hold these units in reserve (so no outflanking). Get yourself three rail-gun armed Hammerheads with Disruption Pods for a 3+ cover save at all shots from 12 inches or farther away.
Tank Hunter Spearhead gives your units the Tank Hunter Special rule so you can have railguns on your Hammerheads at an effective S11 against other vehicles.
Seek and Destroy Spearhead, loaded with Piranhas makes for a pretty nasty tank-hunting Piranha Attack Wing that can scoot out 24 inches and still fire its Fusion Blasters.
I'm sure there are other well-fitting formations for the Tau Empire but I can only see "Railheads" and Piranhas filling up any special formations. Nothing about the Ionhead or Skyray makes them particularly good in a battle with loads of heavy tanks.
The missions are mostly objective based and all Spearhead Formations are now counted as scoring units. If you playing to win games, definitely give yourself at least 3 Piranhas in a Spearhead Formation (Seek and Destroy or Skfall) as this will make them scoring. This will give you a fast and durable scoring unit that can snatch objectives at the last jaw-dropping moment.
The fact that Spearhead Formations are scoring also gives more weight to fielding your Hammerheads with the Ambush Spearhead as this gives you an AV13 tank unit with a 3+ cover save. Its going to take some serious determination on your opponents side to make it through armor like that. Shields Up!
Another thing I like about Spearhead is that the Tau Empire was given a lot of attention. There is lots of new artwork featuring the Tau and some new paint-jobs on studio models. This is important for me to see as it seems that the Tau are still one of the main races in the Warhammer 40,000 universe.