My upgrade to 4.0 went very smoothly. All of the applications worked great, email remained solid and of course, the phone was great. Also, as I am sure you have read from people far more experienced than myself, with 4.0, you can get Verichat, Reqwireless Webviewer and many other great Applications.
If you are thinking about getting any applications, I have included some pointers below, for correctly configuring the apps.
THESE ARE NOT THE DOWNLOADS, BUT RATHER CONFIGURATIONS!!!!!!!!!!!
Reqwireless Webviewer Personal Edition: http://www.reqwireless.com/support_bb_wap.html
Verichat Configuration (POSTED and COPYWRITTEN BY MARK REJHON) BlackBerry Internet - MSN/chat/web/telnet/TCPIP - No BES!
Clearing Email to Optimize memory/mailbox space http://www.tmobile.com/bwc/