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

I was originally just going to get the 4x4 (my other 4x4 was basically and heavy and my hands are very small), but I decided that since I had some interest in all of them, I would get the bundle.

The 3x3, which is the size is a standard 2x2, is very fast with nice turning. I really like the feel and the magnet strength is perfect.

The 4x4 has good performance and I love the size (same as a standard 3x3), definitely my new main. I only have one problem, and that is that it makes a squeaking sound sometimes.

The 5x5 is also my new main, it is very nice. After tentioning it, everything was perfect. The small size is great for me.

Can't go wrong with any of these cubes. Yes the 5x5 is a bit on the small side, but more than functional. I would say these are almost flagship quality in terms of performance.

The 3x3 is okay. But the 4x4 and 5x5 are the cheapest quality. Out of the box, these cubes popped so many times that i am not able to fix them at all. These cubes are very fragile and you should not buy them. Bad choice.

I was originally just going to get the 3x3 for an OH cube, but I decided to get the whole set. All of these cubes are pretty awesome, especially for the price.

The 3x3 is 50mm, which is actually the same size as most 2x2s. It's around 5-6mm smaller than most 3x3s. Even though I consider myself to have decently large hands, I don't think the size was much of a hindrance; it felt pretty comfortable. Mine came with a pretty bad setup out of the box though. It was way too tight and the tensions were pretty uneven as well. After giving the cube a proper setup with some Ångstrom lubes and tensioning, it felt really great! The turning is pretty smooth and fast, the magnet strength is good, and the corner cutting is on par with my usual Maglev WR M. There is a bit of spring noise, but that can easily be fixed by lubing the springs. Another small gripe, the magnets may be a little too strong for OH solving, but the cube's size feels really good. Would definitely recommend if you like smaller 3x3s.

The 4x4 is 56mm which is actually the same size as most 3x3s. It's around 4-6mm smaller than a lot of other 4x4s. Again, the size wasn't a hindrance for me. All I did to set this cube up was tighten it by a turn and a half, as it felt like it would explode if I turned it too hard. This cube legitimately feels like an MGC, but if you scaled it down a bit. It's smooth and slightly crispy, the magnets are good, and it feels very stable. The corner cutting is again on par with my main MGC; reverse cuts go slightly farther, but it has slightly less with forwards cuts. Again, strongly recommend if you like smaller cubes.

The 5x5 is 58mm, which is somehow smaller than most 4x4s. It's 4mm smaller than my MGC, but yet again, the cube's size was not a problem. I tightened this cube a tiny bit because it did feel slightly unstable. This cube very good, but I don't think it's fully flagship quality. It has all the same qualities of the other cubes in this series, it's smooth and fast with a slightly crispy feel. Magnets are once again good and it does feel pretty stable. The corner cutting just isn't fully there for me. It doesn't reach as far, and the corner cuts it can do aren't super effortless. I can do basically just as well on this as my main MGC, but every now and then, it does catch decently bad. I would still recommend it if you want a smaller cube, but it's not as good as other flagships out there at the moment.

Overall, these cubes are all really good! They perform amazingly, and might be the best ones for you if your hands are small enough to use them to their fullest extent. Even if your hands are big, it's really neat having mini speed cubes that aren't really hindered in the way they turn. Their size also makes them really portable if you like taking your cubes with you places. One small issue, the cubes came slightly scratched cosmetically, but you can't expect perfect quality control from cubes this cheap. Besides, it's nothing some plastic polish can't fix. I highly recommend getting these!