Tau are one of the newer codices, and are currently a very strong list. For reasons I can’t really fathom, Tau and Space Marines are Battle-Brothers, so the Tau will fully interact with your marines.
Tau bring the best anti-flyer units in the game to the table. Their bargain choice is the Skyray, which comes armed with six seeker missiles, two networked marker lights, and one additional weapon system. It comes with Skyfire and is relatively tough with AV 13 to the front and AV12 to the sides. Thanks to the networked markerlights, it can act autonomously, which is great as you may not be able to bring many markerlights in your allied detachment. Alternatively, you can take a Broadside squadron with Skyfire and high yield missile pods. This can be paired with a smart missile system, giving each suit eight shots. This firepower will tear through most flyers, and has enough shots to chew through infantry as well. A Broadside team is extremely dangerous to a variety or targets, though it has a cost to match. Fortunately, they’re also pretty durable. The final option is a Riptide, which can take both Interceptor and Skyfire. Though the heavy burst cannon is probably the best choice for taking out flyers, I like the look of the ion accelerator for its ability to also deal with infantry and tanks.
Tau can bring some useful scoring units, though they can’t bring the numbers associated with the Imperial Guard Infantry Platoon. Fire Warriors are a great backfield unit, as the entire squad sports S5 30” rapid fire weapons for a relatively low points cost. Additionally, they can be well-supported by the Cadre Fireblade HQ, who is dirt cheap and allows the Fire Warriors to fire an extra shot as long as they don’t move. Kroot bring extremely cheap infantry to the table, with respectable firepower. They can cheaply be upgraded to snipers (definitely worth it) and can add Krootox Riders, which each carry autocannon level firepower. Finally, a Kroot Hound gives the squad Acute Senses so that they can reliably outflank and threaten enemy objectives. In both cases, the squads perform their role quite well but won’t be as numerous as the Imperial Guard alternatives.
As far as numerous infantry, the Tau definitely lose out to the Imperial Guard. Fire Warriors can include twelve models, while Kroot can include thirty-three models (though 10 of them are hounds that won’t shoot). This is a far cry from the mass of bodies that is the Imperial Guard Infantry Platoon.
However, the Tau really shine in the additional units that the Tau can bring to your army. While the IG options are very one dimensional, most Tau units are capable of dealing substantial damage to a broad range of targets. Broadsides with high yield missile pods and smart missile systems can deal with flyers, infantry, and light vehicles without problems. Even the Skyray can take on a variety of targets. Tau allies can bring a Hammerhead for engaging enemy heavy armor, or a squad of Crisis Suits, which can deal with just about any target if armed correctly. However, much of the Tau support units depend on how large a detachment you take.
Many Tau units provide support only to other Tau units. Mainly, this will be any units with markerlights. While they are tremendously useful, markerlight hits can only be used by Tau units, and it’s possible to include too many markerlights for your Tau units to use. This is particularly true for Pathfinder units, which can bring a lot of markerlights to bear cheaply. Another unit that has this issue is the Ethereal; he produces a 12” bubble of LD 10 and can make all Tau models within that bubble Stubborn, fire an extra pulse shot at half range (particularly nasty for overwatch), fire snap shots after running, or give them 6+ Feel No Pain. These aren’t too great when they only affect one unit, but they’re pretty massive when they can affect a whole army. One model where this won’t be an issue is the Tau Commander with the Command and Control Node and Multi-spectrum Sensor Suite. This Commander allows any unit that it joins to re-roll misses when shooting and Ignore Cover, and he can join any Marine unit since they’re Battle-Brothers. This may come in handy, though he’s probably better off joining a Tau unit since they tend to have more firepower.
Overall, I think Tau are the strongest option for Space Marine allies (this applies to most other armies too). They bring tremendous firepower able to fill many gaps in our own shooting, and do it for relatively few points. Their only failing is the limited number of scoring units that they can bring to the table. Even so, they’ll do more than Tactical squads for the points.