I've used both Mnemosyne and Anki and I have to admit, while I love their rich feature sets, I reallllly hate their interface design and usability. Not only is it unintuitive and overly complex, but they both run really slow on my laptop.
I still can't believe there aren't better solutions available. Anki is probably my most used piece of software and it sucks to have to use a crappy interface.
I think for 99% of usage, there are only a few buttons in Anki. But this may be a case where any sufficiently "simple" interface is insufficiently rich, and any sufficiently rich program will have some interface complexity somewhere.
All that said, I agree. Anki is one of the most useful pieces I software I have ever encountered.
To anyone who is on the edge about using SRS I recommend downloading it onto computer and phone and getting a good shared deck to build the habit. Building your own decks is superior but really, just start.
UI: http://flashcardhero.com/screenshots/ I'm always open for suggestions.