The Dayan Tengyun m offers insane speed that once played with for a while it is easy to control (unlike other cubes) two little problems I have got over have been the plasticy feel of the caps and how lightweight they are. It makes things like U2s feel too easy but still gives it a controllable feeling if that makes sense.
The other common problem found with every Dayan was the aggressive amount of lube found on the cube when taken out of the box. I had to scrub off lube on the outside multiple times before coming satisfied.
THE BIGGEST ISSUE
The biggest issue that put me off was the way you get into the inside of the caps. This was made as if Dayan didn’t want you to get into this cube, as if they had a secret tracker in each cap that they didn’t want you to know about. It was a stupid design where you have to grip two very small pieces of the middle edge and push them down with a screwdriver and whilst putting your thumbnail in between the cap ready to pop it off. Once it comes off I found that I then broke two of my caps due to very very thin plastic. This was very disappointed as I expected a pull design of the caps like various other cubes like the Moyu series.
Other than these problems I strongly suggest the Dayan Tengyun m as if you are not going to take off caps then it is a superb $30 cube worth every dollar. (It feels very similar to gan x with n40s added to the magnets already on the inside of the cube)