Here is my take on them, having never done any trials or worked with them (so you get what you pay for re: my opinion).
DIAM was originally developed by Sabate, which ultimately became Oeno. Sabate was the company that gave us Altec, which was a complete disaster and eventually led to the Sabate family exiting the business, effectively running them into the ground.
Altec was an agglomerated cork product. It was a disaster because it basically did what still makes me nervous about agglomerates: you take your TCA/TBA risk and distribute it evenly among 100% of your batch. In essence, if you had, say, 10 corks, rather than have one cork with 3.0 ppt TCA and the rest with none, you could agglomerate them and have all 10 be at 0.3 ppt. To me, underlying low-level TCA/TBA would be a much bigger concern than a few spotty issues with really bad corks. When Altec took off, Sabate basically lost control of their materials sourcing and started taking shite cork and adding it to the mix. That eventually led to Altec ruining whole runs of wines, and Sabate being sued by a team of producers.
Maybe the super-critical CO2 is the answer? I really don’t know. Sure sounds good, but then so did the plastic corks originally? Yes, they only guarantee <0.5 ppt, which is not 0, but my guess is that is a limit of detection thing rather than any indication of the effectiveness of what they’re doing. Still, though, what happens when you open up your first bottle with a DIAM and get a whiff of TCA? Guarantees of this sort of nature make me a little nervous.
There are enough stories like Altec that I, and you can call me cynical, will probably never be an early adopter of the new “perfect” closure. For the smashing success that is Stelvin (which has taken quite a while to work out all the kinks), there are also Altec, SupremeCorq, MetaCork, the Zork, and a recent thread here suggested that even the Germans are starting to wane on the VinoLok.
This is nothing against DIAM specifically. Just a general approach to closures, being (and always having been) a very happy cork user [soaker].