4MB is probably fine if you only get 20-50 emails a day, almost never fetch attachments, don't use the web, and don't sync PIM data over-the-air. Otherwise get the unlimited service. So if you are a super light user, it's worthwhile to save the $10/mo. Most people need the unlimited service though. You get a whole lot more value for only $10 more (and no threats of overages).
BIS-only ($30/mo) is a new offering. Get it if you don't use a BES server. Otherwise get the $45/mo service.
Yeah it sucks about the text messaging thing. Supposedly the current thinking is that it doesn't cost the carriers much to implement it, but they sure do charge for it. As for sending messages, you could always use the SMS->email gateways if you know what carrier the person is on. That way you could send (At least) messages for free. Tell all of your friends to get blackberries. sheesh. ;) (I'm kidding)
Also one downside on cingular is it would be nice if the SMS messages rolled over each month like the voice minutes do. Maybe some day.