Sometimes mail arrives on my BB before it hits Outlook. Sometimes it is 10-15 seconds later than Outlook. The only downside of BES is that it is another piece of infrastructure (corporate or 3rd party hosted) that requires care and feeding. But it beats the tar out of the other alternatives (BIS or desktop redirector).
BES will do NOTHING for your AOL e-mail. BES is used to push mail from Exchange, Notes or Groupwise e-mail accounts.