I use
Darktremor's a2sd and it works fantastic for me. Keep in mind that if you run a2sd you will likely get very little help in this MIUI forum should you have any questions or issues. For some odd reason the devs here (and I find most devs in general) seem to have something against apps2sd, a2sd and the like. All I can say is that since I installed DT's a2sd my phone has been fine and fast.
One possible downer, which I have not yet tested is that I cannot seem to flash new versions of MIUI over previous versions and must complete wipe every time. I may uninstall a2sd and see if the issue persists.