Folks go on about miui being smooth but personally I found it quite choppy at times, and this was most evident using the miui default launcher and comparing to say an iPhone 4, it looks very stuttery. Its better with launcherpro, but then you've lost half the reason for using it.
I am using cm6 and a theme (minimal matte fwiw ) and its very smooth and nice looking (and believe me I am very sensitive to, and loathe slow performance on a latest Gen phone).
Best compromise I've found yet on android.
Smartphone journey..... HTC Wizard >>> HTC Touch Dual >>> Apple iPhone 3G >>> HTC Desire >>> Samsung Galaxy S >>> HTC Google Nexus One >>> Apple iPhone 4 >>> HTC Incredible S >>> Samsung Galaxy S II >>> Samsung Galaxy Nexus >>> Samsung Galaxy Note II >>> Nokia Lumia 920 >>> LG Nexus 4