Even at this point in time, it is difficult to find NES Classic controllers (much less with extensions) in stock. We reviewed this unit a few weeks after release. The experience has been inconsistent albeit some issues that continue to crop up from time to time.
When fully changed, the controller works decently but there is no indicator (aside from dying in the game) as to when the controller needs more charging. This can be frustrating as a player. On top of connectivity issues (which would take several seconds to resync), there is a noticeable lag. It is most notable on games that you're playing in.
During our review, we noticed that some games had more connectivity issues than others. Mario Bros. 3 and Tecmo Bowl had the most problems, but we didn't have any problems with Zelda. We did notice more issues with more intense twitch like games such as Gradius.
The instructions note that battery charge tends to get low during intense gaming sessions, which then lead to connection issues. This leaves us charging the controller - again. Expect a two hour wait per charge. Once charged, it controls decently with minor issues in lag. From our standpoint, we played for several hours with no issues. We logged in at least 7 hours until we had to charge again.
Ultimately, the Niko Miniboss needs more play time which may mean Niko needs to look at high capacity NmHd batteries to reduce charging times between play.