Plus, I can use Markdown-Here[2] to compose my emails in Markdown.
That aside, it handled those 500k emails way better than e.g. Mail.app (whose performance routinely falls off a cliff somewhere back between 200k and 300k), synced quickly, etc and all without the normal annoyances that keep me away from Thunderbird. A very solid, cheap option.
That said, the re-indexing eventually got annoying enough that I disabled a bunch of IMAP folders in Gmail and switched back to Mail.app. I'd love to find a real alternative, but everything else I've touched has been awful in absolutely-critical ways (usually one or more of: can't handle a couple hundred thousand emails at all, can't display HTML emails, can't stand the UI, or useless/nonexistent filtering tools).