One of the really great things about Aperture, is it's non-destructive editing.
When you are working on a photograph, you can roll back any edits you perform, at any time.
When you perform say, a crop, the crop is not applied to the actual photo (until you export) the crop is saved as an instruction. Let's say I then do some colour adjustment, then realise I do not like the crop, I can go back and re-do it, without effecting any subsequent changes.
The good thing is, you do not end up with lots of versions of the full binary, you end up with the binary with a bunch of instructions on how to render it into different versions, it saves a lot of space and hassle.
The down side is that you need a reasonably fast computer.
It turns out the other downside is that you cannot do any pixel-level editing ...... you can only apply effects to the whole image.
Apple just introduced a plugin API to Aperture 2.1. As the library of available plugins builds up, there will be less need to export to PhotoShop to do edits on regions of images.
Unfortunately, this breaks your non-destructive workflow. As soon as you invoke a plugin, Aperture exports your image with all of your edits applied, before passing it to the plugin, much like editing in PhotoShop does.
The first time I tried it, on a 7Meg RAW image, I ended up with a new 70Meg master, which was passed to the plugin, meaning any effects I applied before the plugin are locked-in.
Maybe it was naïve of me to think that Apple could pull the trick of non-destructive editing in a plugin API. But as they have not, unfortunately, using plugins really throws a spanner in your workflow.
PS. The reason the new Master was so big was that I had the export settings at 16bit, which was probably pretty pointless.