I really like that there are so many clients. Would really like some kind of cloud-sync + mobile client for SuperMemo. It also only works in Windows which means I have to run it under VirtualBox.
My only concern with Anki is whether it still uses some variant of the SM-2 algorithm and whether or not the people implementing the software are doing more than just polishing interfaces, etc. and actively work on improving the learning algorithm. Looking at the history, SuperMemo on SM-11 http://www.supermemo.com/english/algsm11.htm and Wozniak claims that there is a substantial evidence that this algorithm increases the speed of your learning.
Yes, Anki used to use a slightly modified version of SM-5 but the Lead-developer decided to revert down to a modified and improved version of SM-2. Basically he reasons (you can find his explanation on the Anki FAQ) that while versions newer than SM-2 are supposed to (slightly) increase efficiency, it only happens if you study every day and at roughly the same time of day. The reason is, that starting with SM-3 a failure on a certain card changes the difficulty of (determined by the algo) similar cards.
A further explanation by the lead developer can be found here: http://markmail.org/message/u2zfnrg7x53bzp24#query:+page:1+m... I am convinced that it (at least for me) offers much better studying conditions. (Actually, the mere fact that I can revise on a crowded 30min commute on my ipod would make up for any efficiency increase).
My only concern with Anki is whether it still uses some variant of the SM-2 algorithm and whether or not the people implementing the software are doing more than just polishing interfaces, etc. and actively work on improving the learning algorithm. Looking at the history, SuperMemo on SM-11 http://www.supermemo.com/english/algsm11.htm and Wozniak claims that there is a substantial evidence that this algorithm increases the speed of your learning.