I just learned of this feature today.
I have a Galaxy Nexus, and before that a Droid 1.
I've used the mass SMS feature of Android 2.2.x and 4.0.2 many times. All it does is send one SMS to multiple receipents individually. It does NOT automatically convert it to an MMS.
The Group feature is novel, but with so many solutions for this already (email, plethora of IM clients, Google+, etc) it seems a waste of effort to support such a niche feature. When multiple people respond in parallel to an email, the previous is typically quoted, aiding in deciphering out-of-order responses.
Also, this will very quickly make your monthly text message count get out of control, forcing you to upgrade your text plan. Long story short, this feature costs you money when free choices are already available.
Since so many people do not have iPhones (with this enabled) or even Smartphones, the actual number of people that support this is currently quite low, and thus not a feature you should depend upon.
They should first work on getting MMS support in Google Voice.