I have my comcast account set up in BIS. Works great. Don't recommend using the carrier supplied email as the only way to read it is on the BB. No BB data service, no email.
There is also no SPAM filter on it and you can't take it with you when and if you change carriers.
I forward my Charter email to my @att.blackberry.net and also keep a copy that I download to my Outlook when I'm at home. It's worked great for me and Charter's blocked all the SPAM that I get normally.