First off, stay back. It can be tempting to move to 12" and make use of the rapid fire rule on the plasma rifle, but this should only be done when you are more or less certain you can destroy your target (i.e. 2 tactical marines against 3 fireknives). The other time it may be useful is if you are intentionally using the fireknives to draw the attention of the target unit, to protect something more important (i.e. firewarriors, anything on an objective), which brings us to our next point.
Crisis suits are expendable. Don't be wreckless, try and keep them around as much as is feasible, but don't protect them to the point of relying on their survival. They are great for killing, and perfect as a harassment unit, but don't fill up the more important tactical roles of being scoring and being tough.
Anyways, on the battlefield there are a solid number of ways to use crisis suits.
First off, they are aggressive suits. Get a wedge of 6+ crisis suits and just carve a path through your enemy, and sweep them off of objectives. Open up gaps for your other units, and just generally take the fight to the enemy. Manticore in the background making you rage? Introduce it to enough melta to wreck a baneblade. Blob squad causing you trouble? Get some markerlight support, and just chew through them with burst cannons. Most weapons do okay here, the missile pod may be out of place though, not because it wouldn't excel, but simply because you would be wasting it's awesome range.
Next off is defensive, hanging back and popping transports, thinning squads etc. gunline style, so that they are weak by the time they get to you, and then you just thrust away from them. Crisis suits filling this role are generally the greatest source of anti-transports we tau can get, also works well against flyers. Deathrains and Fireknives work well in this capacity, as missile pods and plasma rifles are the only weapons that really lend themselves to this role, due to their range.
Another common way is as a harassment unit. Do not be engaging in full on firefights so much as just wearing away at a unit's edge and boosting away on your jetpack to avoid retaliation. The purpose is not so much to be smashing units, as an aggressive suit would do, but weaking them. The main difference between this and the other suits are that aggressive suits move in for point-blank carnage, and usually have more than one squad teaming up, and defensive units generally stick to their table edge, whereas harassment units may deploy forward or deepstrike behind enemy lines, but generally still work independantly from the other teams and stay an arm's length away from the enemy. Missile pods, Burst Cannons and Plasma Rifles are the optimal weapons for this role, though both special issue weapons may also have their uses in this capacity.
The fourth and easily least common way of running a crisis suits is as a suicide suit. TL flamer, TL fusion blasters, flamer+fusion blaster etc., just to disrupt the enemy and hopefully recover more points than was invested in them, which they usually will, at the cost of a FOC slot. Not often used, and with good reason. I just thought I'd include it for the sake of completeness.
Now something you may have noticed, most crisis suit weapons are suitable for most roles. If you stick tot he more versatile weapons and builds, you can switch roles mid game. An example of how might work is that I usually take my suits (1 team of 3 with MP+BC+MT, 1 team of three with MP+PR+MT, 1 stealth team with 2x FB, most often) and start them out as harassment units, either deepstriking or moving up the flanks, and then once all my suits are behind the enemy, who will almost always be pushing towards your home base against tau, and suddenly pinch in from the rear/sides, using the classic hammer and anvil style with my firewarriors and broadsides and railheads on one side, and the suits on the other.
This also works because it can be varied based on the scenario, for example a green tide is moving up the board, covering enough space I can't move around it. The suits I was planning on moving up the sides can be played as defensive suits, with the deepstriking ones waiting until the greenskins move forwards and open up some room in their backfield for my suits to drop, at which point they would either start clearing the ork backfield of any lootas or what have you, or choosing to move forwards and harass the rear of the green horde.
