this is a question that i asked myself a long time ago... before i had the money to get after effects, aura seemed like a viable way to get a relatively cheap comp app.
but aura is a strange beast of an app.... it's not quite any one particular thing....
it's not really a composite app but you can use it for that. it's not quite photoshop but you can use it for that.
probably the single BEST use for it is as a RASTER ANIMATION PAINT app.... so if you can make VECTOR anims in flash (or even AE), aura will let you do animation that's non-vector... messy, brushy, etc.
but not really good for traditional 'cel' animation like old disney types stuff either though... think the united airline, squiggly line anims or the old dr. katz show from comedy central.
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even if you need that though, i wouldn't recommend aura nor what it evolved to MIRAGE from bauhaus... for what it does, it's waaaaaay too expensive.
if you need this kind of video paint capability, get PROJECT DOG WAFFLE.... you can do a search for it on google... i believe it's substantially less than $100.
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as for other compositing apps, i hear people all the time SWEARING by fusion or combustion or even after effects.
the truth is, if you know ONE app, you know WHAT a compositing app is SUPPOSED to do, the only real reason to learn ANOTHER app is if you yourself need something that your app of choice CAN'T DO or-
- if employment requires it.
that's it. oh, and also if you happen to get your rocks off on just learning different programs (hey, to each his own). and this is pretty much the criteria for learning different 3d apps too.
if you know AE and it does what you need and you're not trying to get a job at a combustion shop, you're good to go.
jin
p.s. oh and since FREE is always good, the free 3d app blender (search it) comes with a rudimentary compositing app if you have the need to play with another comp app....