I've never had problems with InnoDB in the past 1-2 years. In one case it's being used for some fairly computationally intense PHP website calculations.
Many people have used it in more critical applications, and it's up to the task. Slashdot, for instance, or Yahoo! Finance.
It's around the same speed as MyISAM. One thing it's missing is fulltext searching... but you can't win 'em all....
It's not really all that useful for vpopmail, because all the data's in one table anyway. But using InnoDB wouldn't cause all that much harm.
InnoDB is installed and initialized by default on MySQL 4.x. To use it, just ALTER TABLE table_name TYPE=InnoDB (could take a while if it's a big table -- you might want to create a separate InnoDB table and copy a few thousand rows at a time), or when CREATE-ing, CREATE TABLE table_name (blah blah blah) TYPE=InnoDB.
Last i checked Slashdot was running PostGreSQL.