I've found Ifrit to be significantly better than Garuda, mainly because of LoS. You're going to put Ifrit on classes who have to stand still to cast, namely BLMs, SCH or WHM. Garuda can be easily LoS'd by these classes for long enough to get a cast off. And since you aren't microing Garuda the whole time via Place, you pretty much get boned by the AI. Ifrit will chase people down, and is not LoS'able, and if they want to stop to cast, they're going to get hit. If they want to kite Ifrit around the arena the whole team.. GREAT! That just made the fight a 4v3.

Swiftcast->Miasma is definitely superior to anything else. But, with Sprint and Miasma 2, it is generally not even very important. It is pretty easy to run around throwing Miasma 2 on every, but Miasma will last a good deal longer. I find it generally is a good thing to throw on the person you have the pet chasing, especially a healer. It both makes them get hit more, and makes them unable to recover as easily. Swiftcast->Misama and Sic Ifrit on a WHM, and you can pretty much count them out of the fight until at least 1-2 other peopel are dead.

Ruin 2 definitely causes people to miss. Throwing one out on BRDs that are screwing with your healer is a fast and easy way to get some heal spells out more reliably.

I didn't find myself using Fester much at all.. mainly because I wanted to heal myself fairly often (being an SMN, I got targeted by multiple people fairly often) and most of the time it was only ever for 200 potency, so I might as well be doing Energy Drain. The times I did use it were mainly when I needed to burst out a quick 200 potency in combination with ED, or I actually had time to stop and hardcast Bio.

Virus is good to toss out on healers and hard hitters. I'd usually throw it on the class I didn't have Miasma+Ifrit on. And Eye for an Eye is good on anyone who gets focused.

I didn't find myself ever using Blizzard 2, like people were suggesting would be huge in PvP. Nobody stood near me for 2 whole seconds unless they were a melee trying to kill me, and that is precisely the situation where B2 would suck.