The YJ mini 3x3, 4x4, and 5x5 are shockingly good for the money and I recommend all of them if you have tiny hands or are a fan of mini cubes like me.ffF
Here's a review of each of them individually:
3x3
50mm is very tiny but it feels good in the hands. I usually carry a mini 3x3 around with me and previous to this it was a 42mm Zhanchi. However, this cube is better in every aspect. It corner cuts well, it's magnetic, and it has such a nice turning feel. It could work as an OH cube if you wanted it to but the magnets are pretty strong. The actual feel of the cube is fantastic though and if there was a cube with the same feel but scaled up it'd likely be my main.
4x4
Easily the best of the set. Its a bit small for my hands however anyone who has small hands for an adult or is a younger cuber will love this cube. It can compete with the MGC in terms of performance, and is for sure mainable. If you're on a budget and/or have small hands this is a great option!
5x5
Not as good as the 4x4 but still viable. 58MM feels really really small, and going from the 5x5 back to a regular sized 5x5 is more jarring than with the 4x4. Still, it's a great cube, albeit quite fast and hard to control. I'd reccomend putting some slower lube in it and tightening it a bit out of the box. Again, if you're on a budget this is a decent option if you can tolerate the size, but I think that the jump in price to get the MGC is worth it.
Overall:
They're all pretty good, and the 4x4 is very good. Out of the box they all had a lot of spring noise and didn't feel too good, so they'll need a proper setup. If you're a fan of mini cubes I think that these are the best in the market, but I think that only the 4x4 is worth it if you're looking for a new main. They're overall novelties unless you have tiny hands to match or enjoy carrying cubes around with you. That's why I bought them. Just goes to show that its not the size of the cube that matters but how you use it :P