When I first found out about this headset a few weeks ago I was super excited!! The 4 hour playback time was a bit disappointing, but I figured I could live with it for the trade off in discreetness of the unit....
However, further research revealed it can only be used with certain SE phones because the unit is so small it has no volume control and therefore has to be controlled from the phone (and alas, my C905 won't control it!!). It also has no play button (only a pause button), so playback must also be initiatied from your phone.
I guess these are understandable drawbacks when you're dealing with such a tny unit, but it does detract from the whole conveniance of operating your stereo bluetooth headphones with their onboard controls. I use the Sony DR-BT21G for many hours a day, and I can start/pause/stop/FF/FR/adjust volume/answer calls and get 11 hours playback time. That's the sort of things that work for me.
So in conclusion, the Sony Ericsson HBH-IS800 is an awesome effort (and has no immediate competitors) but is let down by some basic restrictions.