1) There are tons and tons of free applications for iPhone. Some of which let you use MSN, Yahoo messenger, skype, etc. 'fring' is particularly good.
2) iPhone is better than every other phone out there, including any blackberry, so long as you can live without an amazing camera, video recording, and mms messages*.
see this review of the blackberry storm:
http://www.nytimes.com/2008/11/27/technology/p ersonaltech/27pogue.html?_r=1&partner=rss&emc= rss&pagewanted=all
3) You can set iPhone to check email every 1min, 5mins, 15mins, 30mins, 1hour, etc. Though really you probably don't absolutely have to have new emails every minute. iPhone also supports push email from exchange servers, and some other servers too.
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* iPhone doesn't currently support MMS. But O2 sends you a text with the address of the MMS photo, which can easily be seen with iPhone's web browser. There are rumours that iPhone may get proper MMS in a software update in the near future.