Some people always tell me to avoid using abbreviations that often used in forum talk, AKA "Internet Slang", in semi formal written conversation. Of course, I would never use the phrase TTYT, TTYL, LOL, YSVW, YW in my email.
IMHO, it is still appropriate to use abbreviation as long as it is very well known and does not contain inappropriate meaning. AFAIR, phrases like BTW, ASAP, FYI, CMIIW and etc. are often appears in my email with customers. YMMV.
What's your suggestion? How do you decide if particular abbreviation is appropriate or not to be used in semi formal written conversation (email) ?