Last few games I've been playing more and more mechanized with my Tau. It works out pretty well, considering the amount of anti-infantry shooting is out there. This last week I added in a 3rd HS tank, giving me Longstrike with a railgun, an Ion head and a Skyray. That's a lot of firepower with a wall of AV13. Add in the Devilfish I run for 20 SMS shots!
Using marker drones instead of pathfinders adds quite a bit of maneuverability and survivability to the marker lights (+1T, +1 armor). I've been moving the commander in and out of the marker units to bolster Crisis suits when needed or markers when needed.
Dropping the gun drones off the piranhas always works out amazingly. Always. A unit that is literally a throw-away unit that can shoot (possibly pin something), contest or score linebreaker is a big thorn in people's sides.
But why does mechanized work? Well, quite simply: Skimmer + disruption pods (or holofields for the Eldar). Moving at all, gives you a 4+ cover save all the time. Need to run for an objective? Flat out gives you a 3+ cover. Combined with the better-then-marine armors of the Tau and Eldar tanks, you have a lot of survivability.
Also, both tanks can do something. Both Devilfish (with SMS) and Waveserpents actually pack enough firepower to be useful, unlike say a Rhino. The Devilfish can completely hide and still fire off shots.
Sure, if someone concentrates on the tanks they will die. But with so many other threats, I've not really had too much of a problem with stuff sticking around.
Also, having guys in tanks really helps negate the menace of the Helldrake.
So a 5th ed playstyle in 6th? Give it a go.