I just sent a test email to my gmail account from one of my other accounts and I got the email within 30 seconds on my BB. I noticed it was slower a couple days ago, but yesterday and today it seems to be back to normal.
Yes, that's an option for instant email. However, you can't delete on the handheld and mailbox when you do it that way. You'll have to go in and clean out your GMail mailbox much more frequently. Also, the @Blackberry addresses can be prone to spam. I had one when I first got my BB but it started getting spammed even though I only used it for forwarding and never gave the address to anyone. I deleted it.