I am using mailmerge to write the same letter with minor differenes to many peolpe. I experienced that switching text paragraphs depending on database values was not easy for me. I ended up putting huge text paragraphs into the database becaus switching did not really work for me.

Actually I dont' understand how writer does it and maybe the boolean evaluation is buggy?

There is some possibility making paragraphs invisible depending on database fields, but it was frustrating. After marking a paragraph as invisible (depending on a condition) it went invisible in the main document and did not come back, I lost the content.

An example in pseudocode of what I want in my mailmerge document:

{if [[balance]] > 10} We owe you money. Please can you send your bank details. {end if}

{if [[balance]] < -10} Please transfer the remaining amount to our banc account 123... {end if}

Maybe this could be done with makros? But how to combine makros with mailmerge?

Can you tell me what are the pitfalls and how to master them?

I once did this with ms word, it was a lot easier. The normal mailmerge (including database fields in the letters) works fine for me in OpenOffice writer.