What MIUI does is build a sleek framework over the best Android build, its basically Android in its finest.

1) Think of it this way, people want to move away from TouchWizz (oem rom) and have a pure google "stock android" rom. Such an example would be going from the SGS to the Nexus S...identical phones, very different software.

This way it performs better because it doesn't have a ton of background tasks and limitations. It also supports more Apps, has less crashes etc because the software is the way it was meant to be.

2) Then comes along the CyanogenMod Team. They use pure stock google android and make modifications. These are modifications to give you more options (in a variety of ways)...so you still get the fastest "Google" experience (and less bugs, more app support) but you have options to do What You Want with it.

3) MIUI is built on top of CM. So it is basically pure google android, but you have access to all the functionality of cyanogenmod. What MIUI does to CM is to tame it.
They have made the UX more sleek, more consistent and even more modifiable. For instance, you go "oh its Friday and install a new theme, it changes everything from the wallpaper, the icons, the notifications and bar and the lockscreen etc etc".

Some people complain saying its too "iPhoney" but they are wrong, theres so many customizability that its actually very different. Besides you can have MIUI and install another Launcher.

Its very hard to explain but pretty easy once you try it. MIUI is for all devices, but its more directed towards phones. There are many tablets with MIUI like Nook Color, Nook Tablet, Kindle Fire, SGTab, etc etc.
The Launcher does come with 4 x 4 grid (suitable for small phones) but you can change that to anything you like (eg/ 8 x 16), that's what I mean about versatile!

If there isn't one for your tablet then you must wait for it, but it might not take too long since the project is now Open Source! Currently they've released the MIUI Apps, but not the actual build (Settings, Launcher, Homescreen, Widgets, Lockscreen -framework with background tasks are unreleased).

Otherwise you can load a CMod (or AOKP) and install the (clone) MIUI Launcher by Tencent, but its only 4x4 grid currently.

If there is already a stable MIUI build for your device, I recommend using it, its worth the risk.


How I rate the different ROMs:
1) Motorola Blur (2.3.3) = 60
2) HTC Sense (2.3.7) = 65
3) SONY TimeScape (2.3.4) = 70
4) Dell StageUI (discontinued)
5) Samsung TouchWizz (2.3.6) = 75
6) VillianMod-TouchWizz (2.3.6) = 77
7) Pure Google (2.3.7) = 80
8) AOKP (2.3.7) = 84
9) CM7.2 (2.3.7) = 86
10) MIUI (2.3.7) = 89
-Hypothetical iOS 5.1 = 80
-Hypothetical Maemo 5.12 = 90

...I'll update the score once we get ICS versions for most of those builds.