Thought I'd give my two cents after using it for a few days:
I work with primarily no-copy items (gacha/breedables), with 1000+ listings. I also have creations of my own that could be listed using the merchant box system, so I've used both that and magic boxes for a very long time.
I... absolutely adore this new system. I went in hesitant, but I've been 100% pleasantly surprised with it.
I find that it's very easy to load things, and associate particular listings (the copy-able ones) with their existing listings. I'm still manually switching over the no-copy ones, little by little. I've noticed a marked improvement in sales - I would go as far to say they doubled, at least with the no-copy items. Magic Boxes have been nothing but a headache since September for me and many others, with a ton of "partial delivery failure" notices that would rack up every time maintenance was done, or rolling restarts happened. Since I started to switch all the no-copy items, I haven't had a single delivery error, even during high stress times, restarts, and anything else that's been thrown at me. I've had three regular customers comment that for the first time in months, LL hasn't auto-refunded their money, or experienced some sort of delivery error, and they wanted to know how I fixed it. Simple: The VMM system. All the unconverted items still have the frequent errors, and I do hope to have them transferred over quickly.
The only bug I've been able to find so far (something I need to document and submit), is that sometimes the names of the listings show up as blank when you first load them into marketplace. A simple "view contents", an edit, and a copy-paste of the name to the listing fixes it, but when there's four or five of them at a time it can get annoying.
Other than that, it's been great.