Here are some little quirks I have found while messing around with the API functionality that will hopefully be helpful for people.
1) When you use MsgBox it doesn't always come to the front when using the code tester. I spent quite a while trying to figure out why my "breakpoints" weren't being reached until I closed out of RE and found lots of message boxes hiding behind the application window.
2) If you run a function on a virtual column it cannot reference another virtual column that is lower in the list. For example, if you have two virtual columns A and B, if A is running a function it cannot reference column B (assuming B is lower in the list than A). The error you get is "index out of bounds" instead of "column not found" when you do something like Import.Fields.GetByName("B").Value
3) If you close the profile without saving (by clicking the X at the top of the window) you lose any code that you added that session even if you clicked "save and close" in the code editor. You have to save both the code AND the profile to keep the code.