Pretty slick, though.
Excellent idea.
To get the very best out of your time for any task you should only have one motive to do it: because you want to.
I'm hoping duolingo comes out with their Chinese track soon. That'll be very interesting.
The best option for learning Mandarin would be Popup Chinese. If you exhaust their materials or find yourself wanting a southern accent, go on to ChinesePod. Once you can already speak and read some, go to Skritter if you want to learn how to write. If I had had these three resources 10 years ago, it would have been a much, much easier path.
My opinion is that SRS must be ingrained with your own interests. Additionally, the base decks on Memrise don't use further flash card techniques other than Q/A or word/definition. Cloze deletion cards come to mind when memorizing things much easier.
This is one of the problems. The framework is good, but content usually isn't. Content in SRS is really hard because language learning should collide with interests in order to feed the fuel.
More dedicated SRS users will just move to something more portable to create their content more easily/personally and beginners eventually drop off.
So far, at least.
As with all these tools, you could teach yourself this stuff before, it'd just require more work and discipline for you to find the necessary books and make your own teaching plan. The advantage of these services is that they lower the friction to learn. Hopefully they reduce the friction enough so that there is a much higher ratio of enjoyment/reward to effort, making you more likely to stick with it. But you still have to stick with it to learn anything.
Here's hoping I will.