I was going to wait till after my next test game against Mephiston before posting on this (as this time I will be the Tau*).  However, as I am not sure when that will be yet and since my view contrasts with some here I wanted to put forth the other side of the fence. (*Note that I have played as the Tau in offline games, but having a batrep to illustrate would have helped, hopefully this will come soon)
GM/ML
IMO this is not a "gimmick" but a core element of the Tau army and one that is very powerful.  But it is all about how you use them.  Which is both a matter of list construction and tactics.  In fact when used optimally they can seem almost broken.  (Note that my useage of ML/GMs involves next to NO attrition-like tactics, no more so than any other army that wants/needs to fling something forward to take out deathstrikes or similar)
Barracudas
Are excellent value INTERCEPTORS.  No, they don't make the best ground attack platforms, because they are not meant to be.  Used as interceptors, along with Tau AA, they make for a formidable air defence.
Fire Warriors
Much better than given credit for.  In combination with the Etheral I reckon it a near no-brainer for a Tau formation to "hold the line".  Personally, I perferred the 2x 30cm shots, as even I don't like getting into CC assault range, but the disrupt shot is useful defensively.
Recon (Tetras/Piranhas)
Both really excellent units, combining to make a really useful formation.  Tetras give you fast MLs and co-ord fire, and Piranhas can stack up AT4 shots on sustain fire cheaply.
Drones
Never used them.  With disposable/expendable or something I might consider them, but as people have noted adding units with lesser saves that still cause BM on kills isn't the best tactic. Perhaps merge regular and heavy drones into one so that they can be up-gunned (and armoured?) and given MLs?
Crisis
I am not a fan of the new Tau Jet Pack rule.  It's too Eldar-like (
) and slows them down too much (well the associated speed decrease does).  I actually loved the old rule, but I am resigned to that not coming back.  But the concept of a rule designed to help the Tau avoid assaults is IMO far preferable to giving the Tau assault boosts (like MWFF etc).
Hammerhead/Devilfish
I have had mixed results from Hammerheads, generally when I am under-cautious they get taken out, but they can also devastate many lightly armoured formations.  The new skimmer rules require considerably more tactical thought to use them, but since the old skimmer rules were an abomination I am more than heppy to deal with that.  Note that I very much doubt I would use Hammerheads as a support formation, if I use them its an AMHC.
But I would say that devilfish are definately too SLOW.  The speed decrease was unnecessary IMO.  My second game with Mephiston resulted in the Tau LOSING the game because devilfish aren't fast enough.  Even if the more heavily armed fish-chassis vehicles need to be slightly slower the devilfish needs to get back to its old speed.
I would agree that for a shooty army the Tau have rather limited access to TK weaponry, either you go for AXs or its a Manta.  And neither of those units is going to make you popular if you need to use them all the time.  Since I assume the Moray was dropped for fluff reasons I am not sure what the solution to that problem is.