One thing that keeps bringing me back to S60 is the number and quality of 3rd party applications available for the platform, ranging from VoIP clients to games and multimedia apps. One of the best instant messaging clients I have tried is Palringo because it allows me to connect with Windows Live Messenger, Gtalk, AIM, Yahoo IM, and more all from a single application. In addition to traditional text IM, you can also quickly send photos and voice messages with Palringo. It is currently available for S60, Windows Mobile, and Java-supported devices and they just announced the availability for BlackBerry devices.
BlackBerry users have been able to use the Java client in the past, but this new dedicated BlackBerry client will offer a better experience for Pearl, Curve, and 8800 series owners. You can setup an unlimited number of "groups" so that sending out a photo goes to all group members with a single action on your part instead of having to send it via email to multiple addresses or notifying people multiple times it is posted on your Flickr site. I don't see the BlackBerry client on the download page yet, but you may be able to point your BlackBerry browser to www.palringo.mobi to get the client.
Palringo is also available for Windows PCs, Macs, and Linux machines so you can use it on your preferred desktop computer as well. Oh yeah, did I mention that these clients are all available for FREE. There are enterprise accounts available too if you are interested in adding this functionality to your company