I like Neat very much. Its basically a stock ROM (which I've always liked) but being deodexed gives it a bit more flexibility for tweaking. Big attraction to me is the developer, who seems to know his onions and is very helpful indeed. Probably the most helpful developer I've seen on XDA alongside Gokhan.
I didnt experience any problems moving directly from GB to ICS. All I did was a full wipe, then flash the ROM via CWM. Do a factory reset and format of /system, or if you want to do a more thorough job, run
GS2ROMNuke
However, I've known a few who have experienced problems with their ICS custom ROM, and found that flashing a stock firmware base through Odin, and then wiping and re-flashing the custom ROM seemed to cure problems.
Personally, I would just full wipe, flash Neat then see how it goes.
If you get problems, then try flashin stock followed by Neat.