Hi chrism_scotland, welcome to Maemo.org.

The N900 is excellent in internet browsing and music playback, you won't miss your iPhone 3G at all, trust me.

The default browser, microB, is awesome and it has full flash support. N900's multitasking manages many pages in background well, and you can browse the web very comfortably, loading many pages at the same time.

The Media Player is good, music playback quality is excellent. Someone doesn't like how it manages albums and playlists (e.g it doesn't follow folders structures, only internal tags, but I think it's a good feature not an issue).

What about IM? Actually there are some services supports, and more are coming. I like how the phone manages IMs' conversations.