I agree with most of the posters, although there are some notable exceptions:
Kroot?!!! Kroot are the main thing keeping the Tau even remotely playable. They're a fantastic unit, as long as you don't bother with a shaper or krootoxes. They can hurt the enemy greatly, have a 3+ cover save in forests, and can outflank. Either use 20 for maximum shooting, or focus on kroot hounds for CC power.
Lone Wolves are alright if you give them terminator armor, 2 wolves, a chainfist, and a combi-flamer. Thanks to the wolves, it's a lot more difficult to take them down through shooting, and having both the chainfist and combi-flamer allows them to threaten anything. Not the best unit, but not a bad one, by any means.
I used to trash Flash Gitz, until a more experienced player urged me to try them out, and now they keep winning me games. Give a large unit of them a transport (either a heavy support one or a trukk borrowed from boyz), all the gun upgrades, and painboy/cybork if you have the points, and you'll have a unit of nobz that can shoot. It's a great versatile unit in larger point games, that opponent's consistently underestimate.
Big Gunz are an artillery unit still worth using. They're dirt cheap kannon shots, in an army that needs them badly.
The Deceiver is worth taking, as few armies are completely prepared for him.
Ogryns are very tough, powerful, and especially useful in larger games, as bodyguards for your infantry blob.
Here are some units no one's mentioned that I can find no use for:
Heralds of Nurgle
Beasts of Nurgle
Boss Zagstrukk (always seems to get his entire unit killed)
Fenrisian wolves (way too easy to tank shock off the board)
Lictors (never seem to have an impact)
Death Cult Assassins
The first casualty of war is neither innocence nor truth.
The first casualty of war is always subtlety.