Here are my two cents for someone who uses both a BlackBerry and an Android. Both have their ups and downs.
BBM in itself is definitely the hands down winner when it comes to IM especially with the D/R, Broadcast, Multichat, Media. I can't possibly type as fast with my keyboard on the MyTouch as I can with my Bold, but that's another topic. BBM is coming to Android, but it's as stated a watered down version, which basically implies, straight forward SMS but with the D/R capabilities. (I assume) I would still use BBM over text just because on countless occasions have the SMS failed to be delivered.
Now the Android, I use all the apps on there, Google Talk, WhatsApp, Fring, Kik, Live Profile. The only one that is widely used for me is Google Talk, a few friends here and there use WhatsApp, but not enough. Google Talk has a really good advantage on android. It updates the chat history as soon as you sign in. Which is a feature I've yet to see RIM put on Google Talk, but I doubt that's going to happen, neither parties seem to care enough for development of the app.
But as far as BBM going to Android & iOS, it won't make much of an impact on the BB side because it's obviously not fully functioning as it would on a BlackBerry. It would just be a way for RIM easily connect their users to Android and iOS users, but with it's limitations obviously. I don't know how this will play out for any of the parties. May be RIM will lose users once BBM has been developed for Android or iOS, May be Android and iOS users will leave to get a BlackBerry. The ever endless rivalries and switching!