I really like MIUI, it's one of those rare examples of a company taking something great and actually making it better. Xiaomi has finally made the entire source code tree available, including the kernel, so we can expect some really interesting up and coming ports. I recently helped a friend install MIUI, he was frustrated with TouchWiz on his Samsung Galaxy S4 mini, TouchWiz was taking up to much memory, an unbelievable 800MB of system memory. He was experiencing lag and major memory problems when using app's as simple as a browser. MIUI, not only made this little guy faster by an unbelievable factor of 10 but the screen actually looked better, so much so that I too just might grab me an S4 or S5 mini and install MIUI on it. Really, I was quite impressed on how wonderful the whole experience was. Oh the system memory, MIUI only took up 380MB, half that of TouchWiz, just amazing. After the install we compared the speed of the now MIUIized S4 Mini to my friends wifes stock S4 Mini, wow, just wow, it was like night and day better, instant computing fast. The UI really is perfect for phones and small tablets like a Nexus 7, even without out all of the gimmicky crap.
I encourage anyone who is fed upped with TouchWiz to give MIUI a go, it will definitely renew your confidence in your phone. I now have it installed in my old Note and it's like a new phone. It's becoming a real problem on how unoptimized TouchWiz, Samsung really needs to completely start from scratch instead of piling on new features just to say, "look, we have that iPhone feature too", just gimmicks and bloat, that's all it is.