I played my first two games using Tau as allies for my space marines yesterday and they performed wonderfully. In a 2,000 point game I ran an 11 man firewarrior squad with emp grenades, two gun drones with a a cadre fireblade, and a riptide with SMS, ion accelerator, and early warning override. In both games I had all my tau sitting in ruins with a thunderfire cannon (improved for a cover save equal to power armor). The purpose of the Tau in my list was to provide a strong firebase to support my two tactical squads and assault squad in drop pods and my fliers. I found that the improved range of the firewarrior vs. a tactical squad gave me the ability to fire at my opponent on turn 1. The fireblade's volley fire surprised both opponents and rapid firing wit the extra shot during overwatch gave me 8 wounds on one occasion. With the riptide sitting right next to them my opponents were rather nervous about getting close.
I don't know if I am sold on the riptide for it's points. The SMS ended up being more useful than the ion accelerator. I played IG and Eldar/Dark Eldar and found that using the nova reactor to double fire the SMS twice was well worth the risk of the single wound. I had to move forward to fire the SMS in order to reach my opponents ADL and then use the jet pack to bound back into cover. He was ultimately killed by a vendetta in that game. The incredible range of the ion accelerator helped provide fire support across the entire board. I overcharged it every time I fired it and with a TEQ save I didn't take a wound on the rare chance I did overheat. He also soaked up alot of shooting that should have been directed at the firewarriors. I think the riptide needs markerlight support to be fully effective. If I could have ignored cover I could have double or even tripled the amount of models he was able to kill.
What I'm not sure of is if I would be better served running something else for the points. A hammerhead with submunitions and TL-SMS and disruption pods is quite a bit cheaper, puts out the same amount of shooting, but may not be quite as survivable. I could use the points I saved to put in a small team of pathfinders and use their markerlights to ignore cover or bump up the BS on the heavy rail rifle.