My Farsight conversion is based on Shadowsun. I used the arms of a stealth suit, regular plasma rifle and shield generator, and an oversized sword from some Confrontation model (i think) that I had used to make a Blood Bowl player with (swords are not allowed in Blood Bowl...). Maybe the model is too small compared to the original Farsight model, and might therefore not be allowed in a tournament.
Farsight is one of the few Tau units that offers some close combat capabilities. With a sword that cuts through most armour he (or she in my case) delivers quite a punch. In a game against Space Marines, he/she slaughtered unit after unit of marines. After all the beating I had taken before then, that was a moment of joy! :-) So I believe that Farsight is a good and fun addition to any Tau army, even though you might not want to use him in every battle.
A cool unit to try is to give Farsight a bunch of bodyguards. It will probably end up in a very pricy unit, and you may regret it if it gets locked up in close combat, unable to bring down it's firepower on the enemy. Regardless, it is a fun unit to try! You can also try out adding a "support suit" to the unit. With a Command and Control Node and a Multi-spectrum Sensor Suite it will boost the other battlesuits shooting attacks, but only if it does not fire itself, so don't bother to spend too many points on weapons for this suit. Also, it's only really effective together with quite a number of battlesuits, and Farsight may take up to 7 bodyguards, so just go crazy with those suits!
It is also nice that Farsight doesn't scatter when Deep Striking, so combine the above with some target locks, and you can potentially take out quite a lot of the opposition in one spectacular blow! For the Greater Good, of course.