Hi, Ronwy ... I'm still fighting the DynaVoxVmax which was delivered last November, so I can't give you much comparison with other units.
First let me note, however, that IMHO the people who are pushing each brand are probably on commission. I may be slandering a lot of fine therapists, but the only time I've seen a wide array of devices available was at an ALS forum I went to last summer where they had a roomful of vendors.
When I went for my speech therapy "evaluation," they only had DynaVox models to show, and it was all explained to me by the manufacturer's rep, not the therapist, who sat back and watched the whole thing. Sounds like the same thing happened to you, when DynaVox was "not available" for review.
I picked the most expensive model with the most possibilities down the road. This is adaptable to eye-gaze technology eventually, so I suppose that is good. However, this thing is heavy (14 lbs), with more wires and peripherals than you could possibly carry around ... keyboard, telephone adapter, other stuff.
All I want it for at this point, since my legs are not too badly affected yet, is talking on the phone. I made a few test phone calls, which were very promising (I picked a woman's voice with a cool British accent .... oh, my, she sounds like cashmere feels! Next, I'll try Clint Eastwood's voice, if he's available.) But then the device went kaplooey ... could not call up menus, could not turn it off ... nothing. I unplugged everything and it still wouldn't shut off ... it switched over to battery :-( ... so I just waited till the battery ran down. So there it sits until I get another "training" session from the sales rep to get it up and running again.
So ... for everyday conversations, I still rely on my Light/Writer. The people I'm around have learned to read quickly and guess as I type, and we can carry on a conversation ALMOST as fast as two speaking people can. When I HAVE to talk on the phone, I tell people to be patient, then type what I need on the Light/Writer, and hold the receiver to the Light/Writer and push "speak." It ain't elegant, but it works.
FROM WHAT I HAVE HEARD, DynaVox is really good. If I ever get mine up and running, I'll give a more informative review! :-D