In your opinion, why is MIUI so good?
There are many tweaks and power features cooked into the ROM. Rooting is as simple as flicking a switch. Updates are available OTA - although to be honest they still want you to download the entire ~210MB ROM :erm:. Luckily this only happens over Wi-Fi.
Biult-in apps such as Monitor, Guard (block calls and SMS), Gallery (much better than Vanilla and Stock), Music Player, Recorder, File Explorer, Backup (including Messages), Compass and Torch (Long press home button in Lockscreen switches it on - brilliant!). I know that these are probably all in the Play Store, but it's nice having it all right there. Oh, and the built-in Messages app includes a pop-up function that I had to install GO SMS Pro for before now.
I like the themes, and the absolute control you have over said themes. Pick font from one, icons from another, lockscreen from still another...
The Notifications area is super-functional, with every toggle you could want out of the box. Hide what you want, use the rest - and a long tap takes you to the relevant settings page!
Power off gives you 4 options off the bat, including Reboot and screenshot (optional).
BLN is implemented, as are little things like Long-pressing back button to kill an app. More in Settings.
All said, it's stable, has all the features I need (I've replaced the Music app with Songbird and browser with Chrome, but Contacts is great - much better than Stock), and I like the launcher (I use an iPad, and was putting all my apps on the homescreen anyway

).
So yeah, it works for me. It's certainly worth installing at least once.
Ok, I'm gonna be flamed senseless now, but, to me, MIUI icons and interface looks butt-ugly.
I'd rather put on Motoblur before I use that.
/runs
Agreed - this has been answered though - there are hundreds of themes out there - and you can mix and match as well...