I've been using the iPhone plug-in for a few days, and my experience has been exceptional. I have an Airport wireless network in my house, and I've been able to watch movies, videos and stream music without having to wait or any delays.
Outside of an initial load delay (which was seconds at the most), I have been able to listen to music, which uploaded and streamed completely end-to-end without interruption.
I have watched videos and movies (again over wifi) from 4:30 minutes through 20 minutes (got bored), but again without any choppiness or loading issues whatsoever.
Going from playlist to playlist, or from song to song, and back-and-forth between pages also has not resulted in any significant delays in either the loading of pages or the loading of the content behind those pages (although I suspect that a feature which allows you to turn off artwork would definitely increase the speed that pages load regardless).
Over the edge network, things are definitely different. Depending upon where you are on the network, you may get inconsistent results. In some places, I've been able to stream music and my songs loaded fairly quickly, but in others, it took a while.
That said, this application is best when over wifi, but passable (location dependent of course), over Edge.
All in all, I think that the iPhone plug-in is well worth the price of admission. Moreover, the DOT.TUNES application itself is a great deal especially since even without the iPhone plug-in, you can get at your music from wherever you are.
If the developer is reading any of these comments, I would suggest a feature which allows you to stream one song after another, and the ability to turn off artwork. Get those down, and I'd buy it again!