[bensummers]

Use one VM per project, not Mac OS X. Unless of course you're deploying on Mac OS X.

Use snapshots and switch between them when disk space becomes an issue.

The big advantage is that, either through careful notes or configuration management, you'll actually be able to recreate your development environment in the future.

[keslag]

Even if you're deploying to mac, you should use a mac vm. If you manage to get a buggy or corrupt package, it's much easier to revert to a snapshot or reprovision a vm than to reprovision your machine.