For starters I have to figure out some basic tactics which led to a debate over the value of the Devilfish, or lack thereof, in the new codex. I know it's pricey, but I think it brings value to the table.
Fiirst off, it's a transport. Now I do run foot-slogging Orks but that's a bit different as Orks are there to die in droves. Fire Warriors are fairly cheap now, but their squad size prevents one from running a blob squad. As the majority of games will involve objectives of some sort, being able to zip out in front in a fairly fast transport to dump off a squad and then provide fire support does interest me.
The biggest negative is it's far from cheap and many a Tau player was surprised they didn't get any more affordable and some of their upgrades got more expensive. For the price of a Devilfish, you can get 2 Rhinos. Of course you do get slightly stronger front armor, a faster tank, the ability to dish out actual fire support (I'd take a burst cannon over a Storm Bolter any day) ad the possibility of a decent cover save. They're a bit tougher and honestly I think they can do a Rhino's job better...well that's not fair as a Rhino is more of a mobile bunker while a Fish is a delivery system
Also about points...a Devilfish is a transport that comes in at about the price of another troop. Tau can now run lots of bodies, though again maybe not as good as Orks but the idea of 4-5 full troops at 1500 points is a definate possibility and with 6th edition's rapid fire rules, you don't lose much for walking. Still with the importance of objectives, the ability to put a troop right where they have to be can be very useful.
What it really comes down to is the battleplan you choose to use. In the fluff the Tau can be "patient hunters" or can deliver a "Death Blow" and the choice of mounting Fire Warriors in fish or not suggests which path your personal Fire Caste will follow.