I don't have any experience, but looking at pictures, I'm not sure why it is being called a bivy. Looks like a double tent. Perhaps they call it a bivy to manage expectations. It doesn't look very tall compared to most tents and it is not freestanding.
4.2 lbs seems a bit heavy for what it is. I guess they don't use high end fabric.... so you could say 4.2 lbs is pretty good for a tent that cheap. I would guess a similar tarptent would weigh a little over 2 lbs... of course it would cost 5 to 10 times as much.
I found an alternative weight for the bivy of 3.75 lbs., so really, I am not sure what the real weight is.
I think that the term bivy is intended to refer to the style of the earlier one man bivys. It is just tall enough to sit upright in. Two people will fit if they each take up no more than 22 inches width.
I think the lack of freestanding capability is to reduce the weight.
I have a high tech tarp-tent by MSR that is decent for one plus gear, but it lacks bug resistance and requires a separate floor. It is tight for two but is about the same as this bivy.
I am in no rush to jump though, no matter what I decide.
Does anyone have personal experience?