I tried it out just now for an album I like. I get about 25% unrelated junk or stuff I've already watched, but the rest are also full albums that seem like fairly solid recommendations based on the album at hand (Jan Jelinek's "Loop Finding Jazz Records").
Start with a random metal track in mind an I find I get a band mix suggestion, a nordic folk metal mix, 11 more tracks from the same band, 3 tracks I listened to very recently, 4 connected other artists and "5 CREEPIEST Old Found Footages - Recommended for you" as a perfect example of the crap being injected recently. They must notice I've never once clicked one.
I do clear cookies and history periodically, so perhaps not having a multiple year history counts against.
In that other link I posted YT suggests pruning your watched history to get better recommendations -- but that's a huge undertaking given the thousands of garbage videos that I've clicked out of in five seconds. Seems like it would be better if the algorithm only operated on data from videos a user manually "likes".
You do? It does ok if I'm watching something technical or some reviews. It's laughably bad at music and wider interest content such as a documentary. If you hit a music track, 50% of the recommendations are the same damn track in degrading levels of quality, 30% the same band. The 10% that's left for other artists are normally a really bad connection to the mood. The last 10% lately is taken by awfully bad match, clearly sponsored, junk though never identified as sponsored.
Most of the time I'd like suggestions in the same genre or mood of music. If I've just watched an upbeat rock track why not suggest other upbeat rock tracks? Last.fm radio was superb at this, for years, until CBS destroyed it. So it can't be that hard.
I think the only times I use the sidebar is if the vid I landed on is terrible quality or a bad hit.