You can in fact keep your old mail server for all users, and use Google Apps Gmail for yourself at the same time, but it comes with some manual administration.
If you choose this dual setup (as I have done in a looong migration phase), then here's how:
Turn on Google Apps and Apps Gmail. The Apps Dashboard will complain a little because the MX record still points to your old mail server and not to Gmail. But it works!
You can send mails from Apps Gmail, and replies go to your old mail server -- where you should add forwarding rules to your new Gmail address. In the Users section of the Apps Dashobard, you can see that your Gmail has the address of
firstname.lastname@example.org. In your old mailbox, set up forwarding to this address, and you'll receive your mail in Gmail, too.
Do that for each mailbox that needs to be migrated, leave others unchanged. Migrated users can use Gmail, others won't ever notice anything.
Only switch the MX record to Google Apps once everybody has started using Gmail.