I've had the Mayflash Adapter for some months now, somewhere before Mario Kart 8 was released. It feels exactly the same as the Wii U Gamecube Adapter but you get no rumble feature due to obvious power reasons. As for the lag, I have none, it all depends on your television and how you have it set up.
Before you buy it, you must know that the adapter will only work on older models of the Wiimote, I have two old models and a recent one, I assume the older models are ones without Motion Plus included but here's a better explanation of compabilty:
After testing different kinds of Wii remotes with the Mayflash GameCube adapter, I realized there are 3 of them so far:
1.The oldest original Wii Remote (Model Product# RVL-003),which WORKS VERY FINE.
2.The earliest Wii Remote PLUS with Motion Plus Inside (Model Product# RVL-036),that contains one of the few serial numbers,which also WORKS VERY FINE:
LMA-RVL-WR/F-C0,LMA-RVL-WR/M-C0,
LMA-RVL-WR/M-C2,LMB-RVL-WR/F-C0,
LMB-RVL-WR/H-C0,LMB-RVL-WR/M-C0,
LMB-RVL-WR/M-C2,LMB-RVL-WR/Z-C2,
LMB-RVL-WR/F-C3,LMC-RVL-WR/M-C0,
LMC-RVL-WR/M-C2,
etc.
(*LAST DIGIT* ending with 0, 2 & 3)
3.The newest & latest Wii Remote PLUS with Motion Plus Inside with an external "SYNC" button on the battery cover (Model Product# RVL-036),that contains one of the few serial numbers which are NOT ALWAYS WORKING:
LMA-RVL-WR/C-C4,LMA-RVL-WR/Z-C4,
LMA-RVL-WR/Z-4M,LMB-RVL-WR/Z-C4,
LMB-RVL-WR/Z-4M,LMB-RVL-WR/Z-C6,
LMC-RVL-WR/Z-4M,
etc.
(*LAST DIGIT* ending with 4, 6 & over)
The first & second Wii remote versions works really OK with the adapter, who makes the GameCube controller acting ONLY as a Wii Classic Controller or Wii Classic Controller PRO with no "ZL button" (the instructions of the adapter are also shown on Mayflash.com for more infos). Unfortunately, when I plugged the adapter with the third & latest Wii remote version, it became incredibly buggy,repeatedly goes in and out of the Home menu, and does other crazy things for 1-5 min sometimes. It seems that the newest version works better with official Nintendo extensions, but is barely compatible with 3rd party extensions like the Mayflash adapter. The RVL# & serial# are shown inside the battery location after removing the battery cover of the remote.
=)