I use chessbook.com right now but ONLY because there is nothing else.
Don't get me wrong, it's great, but I've used spaced repetition in the past and chessbooks implementation is severely gimped.
1. Firstly you cannot adjust the review curve yourself, which can lead to an unnecessarily large workload when 'cards' (I guess in this context that means 'positions') are young, and if you have a tendency to forget things after a larger interval (for example your accuracy after a 30 day interval tanks dramatically) there is no way for you to modify that step to say, 3 weeks instead.
2. You cannot even view stats like that to make such informed decisions
3. There is no way to isolate what are called 'leeches'. In spaced repetition leeches are specific cards that for whatever reason just won't stick for you, so you need to put some extra work in and study them some other way. Chessbook tells you how many 'frequently wrong' positions you have, but gives you no way to actually explore or isolate them, barring screenshotting every 'frequently wrong' review and handhamming the png in some other program. This again leads to a less than perfectly efficient workload and wasted time.
4. The engine evaluations are completely different from what either my local or chess.coms stockfish engine say. Also does not agree with torch, komodo free, or lc0. I have no idea what they are using.
5. Not sure where the database comes from, but the 'masters % picks thing is totally different from chess.coms database.
THAT SAID--I think it is great for now, worth $8 a month *to me*, because using Anki for this would be a nightmare and it has helped me internalize Killer Opening Repertoire and a huge chunk of Jovana Houska's Caro-Kann in just a couple months with about 80% accuracy.
Tldr; bump. Anything to fill this market niche?
It appears that Chess Position Trainer is a dead program since there are never any updates and features are beginning to fail.
What would anyone recommendd to replace it?