For me on T mobile I had IMAP before the new BIS 2.6.
Never had any issues, dont get me wrong it wasnt "push" but it was pretty fast...within 3 minutes. The only thing I see different now with 2.6 is I have a file option and dont have to setup filters. One thing I did do was I didnt set my IMAP up the way you setup POP. Hers what I did . Delete the account. ( doing this will delte your email on your device but they will still be in your messages folder so do a backup if you wanna keep them). Then when setting up the account again just input your email address and do not put the password. It will come up saying thatit wouldnt work but give you an option to input yourself. Follow the prompts to manually input. Your user name is your whole email account...IE me (
[email protected]) then put password. Mail server is imap.gmail.com. Follow the prompts until complete. After complete to make the blackberry diplay the name you want for that particular email account go to edit and change name. This works for me. Hope it helps.