I’m guessing that Vega is my best character.
When using the Flipkick as anti-air, you’ll find that it’s actually quite the superior anti-JUMP move, not so much anti-AIR. Here’s a long example for ya! 
I must be one of the few Vega players who rush down according to the replys so far…
Okay so, if I start a pressure chain, something like j. HP c. LK c. LP bf+LP c. MP, I can do numerous things, like poke again with a c. HP, c. HK, run in and poke (pretty much ALL his ground moves are pokes), hop backwards and abuse your range with tiptoe c. HKs and other walking fakes, blah blah.
Or, you can simply sit there and wait.
If you keep rushing your opponent, they’ll get used to getting pinned in the corner, and if you’re doing a good job on the rush, eventually they’ll get sick of it and start trying to get out.
So now you’ve established somewhat of a mind game with your opponent. You’ve got your Vega sitting in crouch block about a Vega c. MP range away, and your opponent is anticipating for you to continue with a poking string. Now, you can do the safe thing and simply sit.
Of course, if they start trying to rush YOU down, you can counter with pokes of your own to screw them up, then just sit back down again or continue rushing.
Getting closer to the corner now…
They’ll probably try to get out in two ways, one being the roll, easily punished with something like c. LK or c. MK followed by a bf+HP (c. HP Red Impact if you have the meter :D), or they can jump.
If they jump, the Flipkick is fast enough to knock them out of the air before they start an attack.
The thing about the Flipkick is not that it is a good anti-air, because it, in fact, sucks ASS as an anti-air device. It does not have good priority and most good jumping attacks will just eat it alive. All it does is attack the opponent FIRST, before they’re able to perform their attack.
On the HK version, Vega’s trajectory is so high that anything trying to jump over you will get eaten by his feet. The added bonus of it is that it is two hits, which can be deceptive against P/K-Grooves, but I find that I’m using it more as an anti-jump device than a true anti-air, as the speed on the thing is just crazy…
If you want anti-air, try a long distance c. HP, c. HK upon opponent landing (works VERY well at a distance), s. LP in some situations, s. HK at a closer range.
Or just do the smart thing: Parry, JD, or Red Impact. 
But yeah, don’t really use the Flipkick for anti-air unless you’re either feeling brave or you’re using it on reaction to your opponent jumping from a spot right in your face.
You’ve gotta be real fast to beat stuff like Blanka small jump HK or something, but it’s possible.
On the topic of Izuna Drops…
They’re funny as hell to play around with on players who don’t know how to combat it. When doing the Izuna Drop, you have a few things you can do.
A cross-up is the most effective drop… the FIRST time you use it. If you connect with it, a fun thing to do is to Izuna Drop AGAIN right when your feet touch the ground and go for another cross-up, or fake one and then go the regular way. Or, hell, they’re expecting you to attack; why not throw them?
Since some questions have been asked about how to throw them, here’s how I do it:
When I go for an attack, instead of hitting the button when I can score a hit, wait a little bit longer, and push Vega into the side of his opponent so that their chests meet. Hold down+forward/back depending on which side you’re coming in from, and hit punch.
The Rolling Izuna Drop works when you spend a lot of the match attacking with your regular ones, because you can just as easily use the throw option when coming in for the attack (which is more guaranteed damage).
Of course, if your opponent stops one of your jumps the first or second time, stop using the drops. But, if they don’t catch on, or they seem frustrated by trying to stop you, do NOT stop using the drop…
You’ll be surprised how much damage you can get off of these alone with the 3 attacks you have (those being cross-up, fake the cross-up and go front, or throw). The super version is pretty much just for fun, but if you can get your opponent into the habit of blocking your dives, then you can probably land the grab.