FWIW I have now upgraded (IMHO) 2 servers from postfix+dovecot to a full courier-mta+imap system and so far I haven't noticed any drawbacks in day to day usage. I expect some problems when upgrading ISPConfig because it's scripts expect postfix to be installed and in my case I had to remove the postfix package altogether so Ubuntu would install the courier-mta package. A workaround may be to simply leave a copy of /etc/postfix (and maybe some other) directories so the upgrade just thinks that postfix is still available.
The main reason for me to bother is that I was not happy with the default spamassassin spam filtering system and not being familiar with it, or postfix in general, it was easier and quicker for me to install courier-mta and use it's maildrop local delivery agent with Dspam to get near 99% effective spam filtering.
On Ubuntu (13.04) this is the magic glue that makes it work with the ISPConfig MySQL auth tables, /etc/courier/authmysqlrc...
MYSQL_QUOTA_FIELD concat(cast(quota as char),'S')