I have been an annual member of PianoMarvel for last 3 years at least. My two kids and I all use it. And, have been very happy. Usually, they have a sale for reduced annual price around ThankGiving/Christmas.
It has multiple components: Sight reading, Learning Repertoire, Technical exercises and has SASR test (that checks your sight reading skills).
Re: technical exercises, I have only used a few Czerny exercises (I haven't used scales & arpegios) - but they are very helpful. Especially, as you work on improving the tempo.
Re: Sight reading, there are sight reading boot camps and also other sight reading resources (please see my other post with some links). It is the best resource I have found to improve sight reading. And, if one puts the time in, it will definitely improve one's sight reading. I also highly recommend taking SASR test - just to see how well (or poorly) you stand right now and repeating the test every 2 weeks or so.
Re: Learning repertoire, this is nice - they have many common pieces in their system. And, I like that you can set different tempos and test your playing. You can split LH/RH and you can also split into sections (3-4 bars etc.). And, there is also a way to take a musescore generated XML file and upload your own pieces in the system. This has been very helpful for my kids' recital pieces which aren't there already. It is a bit of a learning curve to upload your own music - but at least the option is there.
They also have "method" based learning where you go through a sequence of repertoire pieces in increasing order of difficulty. We haven't used it as we follow the piano teacher's method - but this could be very helpful for self learning.
The website is also continuously improved. They used to have a terrible plugin based system to connect DP via Midi - but that has been removed and it works without any plugins. The customer service is also pretty quick - I contacted them a few times about wrong notes in some of the pieces and they got back to me in a day or so.
The only criticism about the website is that it doesn't really check if you are holding the notes for their required time (e.g. a half-note should be held for its full time). Or, it doesn't check for pedals. Or, it doesn't check for dynamics. Or, it doesn't check for, of course, 'musicality' of playing. For that, there is no substitute except a good teacher. Basically, the website does a good job of checking that you got the right notes at the right times.
I have only used it with Digital Pianos. I don't know how well it works with acoustic pianos.
Overall, it's definitely worth trying out if you have a DP you can connect.