Imperialtrader...that's just simply not true. Pulse rifles are the biggest threat to Space Marines in the Tau arsenal, and don't let anyone tell you otherwise. Anything that wounds you on a 3+ is a serious problem, even if you get your armour save. It comes in enough volume that you're going to be taking quite a few of them, too.
The Tau are, in my opinion, rather hard to beat if there's an even halfway competent player at the other end of the table. The main weakness of the Tau army is, obviously, close combat. That being said, it's very easy to keep Tau out of combat. Devilfish models are such that you won't be able to assault around them in a single turn, so you'll take a lot of rapid-fire pulse rifles, and more often than not, twelve fire warriors can pull down the one or two Space Marines who manage to survive the torrents of fire that get poured into the nearest melee threats.
Plasma rifles for the Tau don't get hot either, and they're very inexpensive, so you'll be seeing a fair number of them. Crisis suits typically mount the "Fireknife" configuration, which has a missile rack (which is the Tau equivalent of an autocannon) and a plasma gun. Crisis teams armed as such will jump out to rapid-fire your squads closest to being in melee range, and then leap back OUT OF YOUR POSSIBLE CHARGE RANGE IN THE SUBSEQUENT TURN. You will be very hard pressed to engage Crisis teams in close combat, so you're going to have to shoot them.
The trick for fighting the Tau is to be aggressive. If you present more immediate threats than they can counter, you can eventually get some depleted squads into combat. You'll take monstrous casualties on the way in, but if there's no obvious target for them to engage first (i.e., you don't go after the Tau in waves with assault squads first, followed up by mounted units, and then the third wave of tactical squads and foot-slogging Command squads) you should be okay.
It may sound utterly insane, but Space Marines CAN put up a rather good firefight with Tau if the army isn't loaded with Crisis suits, Stealth Teams, and Devilfish-mounted squads that are positioned in a manner that they're impossible to assault due to game mechanics. A traditional firing line, which occasionally does turn up, can be countered by your own firing line of Tactical Squads. Statistically, interestingly enough, a Tactical Squad has a slight edge over a Fire Warrior team in a firefight. Tau hit on 4+ and wound Space Marines on 3+, while you have a 3+ armor save. Space Marines hit on a 3+ and wound Tau on a 3+, and they have a 4+ armor save. The numbers are slightly in your favor there-- but be aware that there are more fire warriors, and thus more shots coming your way. If there are static units of Fire Warriors in a traditional firing line, high-volume AP4 weapons work wonders-- heavy bolters and assault cannons are excellent tools for fighting the Tau.
Your armour is forfeit against the Tau-- you might have brought it with a purpose in mind, but the railguns on the hammerheads and Broadsides, combined with the rather surprising array of other anti-tank weapons the Tau can muster (Pirhanas with fusion blasters, and the all-rounder fireknife Crisis Suit) will make mincemeat of it. It's better to position it to provide cover for your advancing infantry in the form of wrecks. If you can shoot with it, that's great, but your overwhelming objective is going to be to deliver infantry to fight in close quarters in whichever manner you prefer.
Incidentally, ignore the broadsides. They're always deployed in the backfield and you're going to have enough trouble getting to the front lines of the Tau army. Their 2+ armour and the inevitable shield drones that accompany them make them an extremely tough nut to crack, so it's better to just let them do their damage and focus on threats you can actually neutralize.
