I'll chime in here, but of course I am biased as this competes against what I sell so please keep that in mind (::: cough, cough, their plan SUCKS...cough, cough).
Anyhow, this is Cornerstone's standard plan, and Dinan became a selling agent and had them tweak coverage to include Dinan mods. Remember that mods have to be not only purchased from an authorized Dinan shop but also installed by one. For many people that's not a big deal, but I could see a future "Uh oh" in a claim if you did not pay attention to that clause.
The rest of it is standard Cornerstone stuff. As a warranty administrator, their reputation is not stellar. They are by no means a rip-off or anything, but certainly not in the league of Fidelity, Zurich, EasyCare (although EasyCare's reputation has dropped off a little lately), AmTrust, etc. I don't want to knock a competitor though, it always sounds like sour grapes.
If you have a heavily Dinan modified car, or plan to, and use the Dinan installer network, this plan could save you from possible hassle at claim time due to modifications to your car. I think it's a smart move for Dinan. Beyond that, apples to apples with the top tier companies, to me the adminstrator is a step down.