So I've finally gotten these things out to several pickups and 2 games. Ultimately, I feel they do their job, although they don't suit my style as well as I thought.
The plates I got are second generation, meaning they are made with steel screws instead of plastic. This helps to make them a little more solid, since there was a lot of talk of the plastic ones simply popping out. If you're looking to buy, definitely go far 2nd gen or later. They are sturdy, but I think a direct puck could break them.
They add a decent bit of slide to the court, and a halfway decent butterfly slide becomes a reality when playing roller. It's very nice. Some guys estimate it's 80-90% as slippery as ice, but I didn't find that was the case. I noticed I was having to push much harder to get the same slide across I would have gotten if I was on ice, so I'd put it roughly at 2/3rds or so.
The biggest advantage of these, at least for me, wasn't butterfly sliding, but simply being able to flare legs out and move around once you were down in butterfly. I made a save on a deflection one game low glove side with a quick kick out that I wouldn't have been able to make without the added slide ability.
However, my dislikes are that they add noticeable weight to your pads, although that's to be expected. They are easy to attach but more difficult to get to stay reliably in place; mine tended to shift little bits throughout the game. I've heard recommendations of industrial strength Velcro to hold them in one spot, and I would go this route if you were serious about keeping them on permanently.
My main gripe isn't so much a fault of the product, but rather an incompatibility with my style. I simply play too much of a hybrid style for these to work for me. On both ice and roller, I strap my pads a little tighter that a strict butterfly goalie. The lack of pad rotation hurt me a little here, as it made it tougher to get the plate perfectly aligned for a quality push. I also feel I rely too much on half butterfly, and don't rely enough on butterfly slides to make these live up to their full potential for me.
Overall, I'd recommend them to hardcore butterfly ice goalies looking to play roller. For hybrids or long time roller goalies, the benefits simply aren't really there.