The Tau are a regional power at best. I find them quite weak on the tabletop, which is why I changed my main army to Chaos Marines (I love the models and concept, but it's hard to have fun when people are yawning and just tossing Sternguards on drop pods behind you and frying your vehicles with Land Speeder MM like it was going out of style), but in the fluff, I see them as a growing influence mostly due to the favorable situation in the galaxy.
The Empire has its hands full avoiding societal collapse, pancking over a flickering Astronomican, dealing with tyranid mega-invasions and whatnot. It could likely muster enough power to crush the Tau for good easily, but that would leave its defenses against other threats depleted, even assuming all goes well. The Tau would have to work overtime for 400 years to claim half as many worls as a splinter hive fleet would.
The Eldar seem to have a bit of a soft spot for the Tau, though that may change is the lovable commies claim a few Maiden Worlds.
Chaos is a lighter threat for them: they don't have psykers flaring out of control and allowing daemons to pour into worlds, and chaos marines apparently focus their attention of races with a better warp presence that can please their deities.
Tyranids are as big a worry for them as for anyone, but galactig geography actually helps them here: their domain is tightly packed and easier to defend than scattered systems with vast emptines between them. They would collapse easily if in the path of a main invasion fleet, but can deal with lesser tendrils efficiently.
So summing it up, they are a smaller power that can sometimes punch above their weight, and the current context certainly helps them do so. And every great power starts as a smaller, weaker force that could have been stomped easily by the previous overlords...if only they hadn't been so busy tending to theor overlordness.
As for the Tau haters...I think they are just peeved that Tau fashion lacks the abundant skulls their favorite factions have.