Now that the Mystical Skyfire Diamond now walks with a bit of a limp, this sort of thread is an excellent reference for selecting something else to toss in your Collar of the Uber Mage. Basically the commentary goes something like this:
Mystical Skyfire Diamond: The shiny toy of yesterday is a little less shiny now, since the 5% chance for spell haste has been changed to "chance on spellcast" which works out to be about 2-3%. That's not to say that it's not a great gem, only that it's not as powerful. This gem now works on Evocation, so is most likely better for the Arena mage than the raiding one.
Insightful Earthstorm Diamond: Deemed virtually useless in most cases, this meta gem has a 2% chance to increase mana regen. Of note is the fact that there is no hidden cooldown for this gem, so you have a slight possibility of multiple procs.
Swift Starfire Diamond: Although this meta gem gives you both +12 spell damage and a run speed increase theoretically allowing you to put more stamina on your boots instead, it's lack of anything unique makes this a waste of a meta gem slot.
Powerful Earthstorm Diamond: Having +18 to Stamina is always a good thing, but the +5% stun resist will pretty much be negated by the Blink reflex. Still, there isn't much out there that gives mages a plus to stun resist. If you run across one, cool, but honestly Stamina can be gotten elsewhere.
Chaotic Skyfire Diamond: Get ready to spend lots of time in Shadowmoon Valley hating on the naga. The "exactly 2 blue gems" rule on this meta gem will soon be changed to "at least 2," making this a better gem than the MSD. Not only do you get an increase to spell crit rating, but also a bonus to crit damage. While the crit damage boost is not the most fabulous proc out there, every little bit of extra damage counts, and hey, there's no "chance on spellcast" nonsense.
While Andúnë focuses his research on meta gem viability for PvP, I think that his analysis translates well to the PvE realm as well. Looking over the list, the options are actually fairly slim, and in my mind it all seems to come down to the new meta gem, the CSD. Perhaps they will up the drop rate on the design, because that Coilskar Siren just became the most popular mob in Outland.