Unsolvable Riddle (the Tol Barad trinket) is good, but apparently the ilevel 346 trinkets you can get from heroic dungeons are as good or better, in part because, like Greenbeermonkey says, you don't have to manually activate it, but also because it averages out to be less of an agility boost than Tia's Grace or Grace of the Herald hc (266 plus a chunk of mastery vs. 340 plus less mastery and 285 plus some crit).
If like me, you're running heroics everyday and the trinkets just aren't dropping, feel the Tol Barad one, but if you then get Tia's Grace and Grace of the Herald, don't be afraid to use them instead. The upcoming alchemy trinket will also be very good.
As for Fluid Death, it's very good in terms of agility, but it's value will be lessened slightly if you're not using all the hit after reforging. Unlike for a caster, hit is still useful after the 8% cap, just much less. The trinket will probably still be worth it, but something like www.femaledwarf.com
will help you figure out where you drop the excess hit if necessary.
And for the Cobra/Steady haste thing, they've mentioned that the way it was phrased in the patch notes is misleading - presumably they're changing how those shots scale with haste.