Ive just flashed 1.7.x to 2.3.16 on my S2 and first of all I want to thank the developers for putting in the time to get this excellent rom up and running (and ofc props to the CM9 developers).
Had no real issues with the update (just wiped everything besides my sdcard), besides the obvious (no MHL and poor cameradrivers, which I blame on samsung not opensourcing those), I did notice a few small issues (not blocking):
1. according to my system monitor only 1 core is shown/being used (system panel), it does however detect 2 cores.
2. even with forcing GPU/hw rendering for everything it can feel sluggish, this primairily happens with the MIUI launcher (go launcher seems snappier for example).
4. battery drain is slightly worse compared to the 1.7.x GB rom I ran before.
5. it seems to have more issues with setting the proper light intensity if I enable automatic brightness (and with issues I mean that it randomly goes too bright when there is enough ligt and tends to show small fluctuations with stable background light)
6. not everything is themed yet, which isnt really and issue since it doesnt hinder functionality
I also wonder if there is a bugtracker for miui specific bugs, I cant seem to find it on this site or on
http://en.miui.com/
Anyway, keep up the good work