Deep Striking: Tau do it best

1. Delayed Arrival. We can't all be GW's poster boys, immediately arriving on turn 1, impervious to mishaps. Our reserves arrive as early as turn 2 and there is a very real possibility they will arrive as late as turn 4.

   I think Tau definitely need Drop Pods...

2. Can't Assault. That is a whole phase of damage inflicting potential that you are missing out on. Not a huge deal for Tau, but our suits and Kroot really aren't that bad in cc.
3. Exposed to return fire (Deep Strike). Usually you'll try to drop in a safe place, close enough to either charge next turn or rapid-fire this turn. That means you are standing in the furnace and if you don't make this shot count, you'll be dead next turn.
4. Intercepted. Tau are exceptional controllers of reserves, even your opponent's. But some factions can Intercept your units as they arrive, and that can be a danger if you are using fragile attack platforms
5. Loss of Firepower. Anything you put in reserves can't help you on turn 1 (excepting aforementioned auto-awesome-cuz-I-spess-mahreen's). Games can be won and lost in the first turns of the game, and giving up firepower on the front end can put you on your back foot, playing catchup.
6. Mishaps. The biggest danger, as there is a 66% chance your unit will either be dead or hopelessly mispositioned. The other 33% chance is that your unit goes back into reserves and that is another turn lost.

The Benefits

1. Cannot be targeted. With the advent of ranged D weapons, Centurionstars, mega-alpha-strike armies, and other super killiness silliness, we have reached a point where placing a model on the table will spell certain death if you end  up going second. This benefit wasn't such a biggy before, but now it is a very real advantage that your units cannot be targeted in reserves (yet...I hear space marines still need a 7th edition codex).
2. Hit First. You get to shoot first, barring any Intercept shenanigans. Even if you lose the roll for first turn, or intentionally go second, you can deploy your models to get at least one maximum firepower shooting phase. You have to weigh this benefit against not shooting at all until they arrive, but for close-range weapons this is a clear winner.
3. Reaction time decrease. You get to see the beginnings of your opponent's plan in the first turns. You can bring your models in exactly where they need to in order to undermine ore counteract your opponent's strategy.
4. Equivalent of extreme mobility. Regardless of how you are arriving, you have a big area you can arrive in, and you get to pick where (for the most part). Not only do you get to react to your opponent's moves, you don't have to worry as much about these units being drastically out of position and unable to get where you want them. 

   Can't Catch Me!

Why Tau?

1. Thrust moves. We can use these after a Deep Strike to re-position our battlesuits. This is huge, since counter assault and blast templates are a very real possibility and this helps with that.
2. Multi-Wound Deep Strikers. We can deep strike into forests and on top of buildings, and we don't have to worry about losing any of our firepower once we arrive! Sure, losing a wound sucks, but even if every suit fails the dangerous terrain and armour tests, you still get to shoot at full capacity! The exception here is Stealth Suits, which should probably be deploying via Infiltrate or Outflank anyways. Vespid also have only one wound, but they auto-pass dangerous terrain tests.
3. Powerful Shooting. Our Deep Strikers pack a punch. ALL of them. No wasted bodies on bolters so that you can take meltaguns because EVERY battlesuit carries 1-2 of the weapon of your choice. 
4. Spammable. We can bring in a LOT of reserves, and you don't even have to spam specific units. Crisis suits, Stealth Suits, Kroot, Darkstrider+Firewarriors, Piranhas, Flyers, Vespid are all good reserves options.

   Crisis suits dropping Everywhere!!!

What to use

1. Kroot: Infiltrate and (subsequently) Outflank. You even get to put them in Outflank after you've seen both armies deployed. When carrying snipers they can hurt most things and they are difficult to root out of a forest. They also move quickly to cover ground and grab objectives.
2. Crisis Suits: the tip of the spear. Perfect for Deep Striking and not too pricey for the damage potential they have when supported by markerlights. Especially useful when running a Farsight Enclaves detachment, as you can take way more of them and they become Obsec. A single Crisis suit with two flamers is a measly 32 points. He's no Scat Attack Bike, but he gets the job done and will surprise you with how resilient he is.
3. Stealth Suits: Deep Strike AND Infiltrate/Outflank. Don't Deep Strike them. Outflank or Infiltrate will get them where they need to go safer. Outflanking them is fantastic since they are massively hard to kill in the later turns of the game, especially if they come on where you want them to.
4. Flyers. Naturally, these have to arrive via Reserves. I don't use them myself, but they can be instrumental in countering enemy aircraft and dropping Ion blasts on enemy infantry. 
5. Fire Warriors. Arriving from regular reserves will keep them alive until you need them. Best used for grabbing backfield objectives and denying your opponent first blood off of our relatively squishy infantry. If you take Darkstrider and a Devilfish, you have a decently mobile Outflanking Obsec force that will wound just about anything that isn't a vehicle. Perfect for storming your opponent's backfield objectives.
6. Farsight Bomb. Of course this must be mentioned. The Farsight Bomb is the Tau's most powerful unit combination, it puts out the maximum amount of firepower with maximum flexibility against all types of targets. It arrives via Deep Strike and does not scatter. If you build it right it also ignores dangerous terrain, can almost never be caught in combat, and is incredibly hard to kill at range. This unit is less for disrupting your opponent's advance and more for breaking in his teeth and killing everything within 18", hopefully winning you the game.
7. Comms Relay. This is a must for any Ninja-Tau commander. It is all we have left for manipulating reserves and is relatively cheap (70 points with an Aegis defense line). You can use this to try and delay some of your forces as well, to give your opponent time to spread himself out and open up firing lanes.