So far, so good with MIUI v2.6.1 (Android 4.0.4) on my HTC Evo 4G.
The interface is simple and beautiful. The settings are straight-forward and approachable. You cannot change the configuration of (literally) everything, like I could on Cyanogen 7...but I'm fine with that. Gorgeous and easy is at least a good set of tenets for a mobile OS.
Their music app is really fantastic. I love that it is very similar to the music player on the iPod Touch. It also has a simple 5-band EQ, which is sweet. You can set it to auto-download album artwork if you like, as well...though I disabled that and just use art embedded in my music files. It has no issue with all the file types I use: M4A, OGG, and MP3.
The dialer, the contacts app, and the texting app are all very well done. The lock-screen is also fabulous...not only showing you if you have messages, but also changing the color of the lock-button if you running out of battery, or are charging, as well as the charging progress. The whole lock screen also changes to the album art with controls for the music player when the music player is doing its thing.
I don't have a 4G outlet in my area...but WiFi and 3G antennas have worked perfectly. Also controls for turning off and on those antennas, along with you Bluetooth, ringer, and even flashlight...are all right in the pull-down notification area.
The only apps that I haven't been very impressed with are the camera and the web-browser. The web-browser, only because it doesn't zoom in on text very well...in that it doesn't reformat that text for the small screen. Perhaps it just isn't sometime I've figured out how to config? After all, I've changed almost nothing from its defaults. The camera is fine, but it just doesn't have quite as many controls on screen as I'd perhaps like.
Overall, I think that this build of MIUI is fantastic. If you want to control every little thing and config on your phone...then this mod isn't for you. But if you like simple, beautiful, and very usable...MIUI is great.