I got one of these things (a bit of a surprise I must say) and been trying a bit. It booted up every game I tried (around 12~15) except Gradius V.
Games like God Hand, Romacing Saga, Dragon Quest 8 and FF12 IZJS worked fine to my surprise, the Katamari games also work (thank goodness) and other smaller ones like Mr. Moskito or the Contras too. Videos play fine (I seriously did not see any difference from playing them from disc, but I'm sure it could depend on the quality of the video itself, most games didn't have super high quality videos though) and games run well. Of course I haven't put much time into each one yet as I only got it two days ago, but I've read people left games running for 14 hours and they didn't freeze or lose connection to the MX4SIO, so that sounds great.
It's rather surprising how well it works considering it is reading games through a Memory Card slot, specially when looking at the USB slots right next to it. Makes no sense to me, as at most the system would need to read mere kilobytes out of a MC.
I have a PS2 Slim 75004 model by the way. I did have no issue with having to eject and insert the SD card for it to read games at all, it has been very solid, but of course that will depend a lot with how the device was made; some of these things seems to be only "slide the sd in" while others have that "click-y" slot that keeps the sd card in place.
From what people say in psx-place, Sandisk cards do not work at all. I used a Samsung Evo Plus 64GB and that worked fine. It is recommended to format it FAT32 with 16kb clusters.
Of course the question will be if OPL will get worked on with some level of consistency. Right now the most important thing would be either fixing saving the configuration of OPL on the BDM3 version (which can save/load games from the MC on port 1) or making saving/loading games work on the main OPL version, right now 1.2.0 I think.
Having to activate the USB/BDM load every time you open OPL is a bit annoying, but hey, for now it'll do.
It definitely is a large step up from USB loading. Games that I never got to boot on USB, play fine on these thing. I don't expect every game to work, but just being able to play something like those long rpgs like Romancing Saga above or those replayable quick games fine through an SD card is awesome and will take a huge load off the disc reader, because those things can fail in many ways beyond the lens getting worn out.