Back when I used Netflix primarily for DVDs, the recommender system worked pretty well for me.
Much later, when they switched to simple thumbs up/down, the recommender system was entirely useless to me. (Not merely because of the dumbed-down rating system; the recommendations were genuinely bad.)
For the time in between, I'm not sure if the degradation was gradual, sporadic, or not degraded at all.
It's pretty clear that their recommendation system and broader UX is designed, at least in part, to obfuscate how much content they have and how good it is. Back when it was DVDs and they had basically everything it was more about finding the next best thing for you. Now it's finding the next best thing they have and, preferably, something they own the rights to.
If you add dvd subscriptions you can still get the original Netflix recommendations back, including sorting by top predicted rating (which to me is eerily amazing). I used to keep the dvd subscription mainly for the recommender, with the delivered blurays considered an extra bonus for very rare movies you couldn't stream even if you wanted to pay.
Much later, when they switched to simple thumbs up/down, the recommender system was entirely useless to me. (Not merely because of the dumbed-down rating system; the recommendations were genuinely bad.)
For the time in between, I'm not sure if the degradation was gradual, sporadic, or not degraded at all.