I haven't used the Plantronics 910. Apparently it's compelling feature is that it can accept connections from more than one device at a time. (Most Bluetooth headsets to be paired with a single device at any one time. So, it's either connected to your computer or your cell phone, for example.)

I have played with my Jawbone headset on my computer, though, placing calls through Gizmo Project.

It's a pain in the butt to switch back and forth between a cell phone and the computer, so if you plan on doing this, I'd definitely recommend going with the 910 or similar headset.

Yes, you can hear music, but it's going to sound awful and be monophonic. This is a function of the limited bandwidth offered by the Bluetooth profiles used (Handsfree, Headset). No headset using these profiles is going to give you good audio quality for music.

Yes, you can send mic output to a speech-to-text applications or any other audio application that uses a mic.

The above two things are a function of the Bluetooth stack running on your computer. The above is true for the Widcomm stack - not sure about others.

If you want to listen to music, get an A2DP headset. These will typically also support handsfree and headset profiles.

A2DP transmits a compressed, stereo stream. It also has considerable buffering to protect against dropouts. (Not great for watching video, because of the buffering delay.)

An A2DP headset will switch profiles when handling a phone call. Watch out, as some are pretty stupid about how they do this, and only route the signal to one ear during a call. Kinda dumb, but many work this way.

I don't think there are any A2DP headsets that support multi-point like the 910. It will have to be connected to either your computer or cell phone at any given time, and you will have to fiddle, fiddle, fiddle to switch it.