I talked to Zack about the sale and a number of interesting things came up that I think are worth noting. For one, it's based on the dsIDXPress plugin — which it turns out is by Diverse Solutions, a Zillow company. While the reviews are fairly negative (for the dsIDX plugin, not IDX+, which seemingly has happy customers), they seem mostly support related, not product related.
Also, the whole discussion just reminds me how big of a world there is out there of paid plugins. IDX+ was a pretty good product for Zack, but he wished to focus on his quickly growing GravityView plugin instead. It amazes me the various niches WordPress reaches.
Finally, I just think it's worth noting how Zack and Bruce are handling the switch. For one, he was happy to sell to a customer to keep the plugin in someone's hands that had a vested interest. But also, they made an announcement on the product blog and are planning a webinar to address any customer concerns.
I think it's very well done, and I enjoyed seeing how this is all playing out. It seems like it could be a good deal for all involved.