I use DS980 with my Kaiser (HTC TyTN II) with no problem whatsoever. It seems to pair better with Kaiser than Hermes.
It is able to read incoming call as well as call history from Kaiser without the need to install the Jetware HF extension (which is required for Hermes).
Voice dialing with the build in cyberon software also works great.
Regarding the volume, it is as loud as it is supposed to be. In fact, when set to max, it is too loud for me. I need to reduce the headset volume to make it comfortable.
All devices paired to this Headset will have their BT names shown on the LCD. Thus you can easily check how many devices have been paired with it. Moreover, you can easily switch the connection to the device of your choice. You don't have this feature in the DS970 model.