My worst purchase was master of orion 3. something like 3 years in making and it was a complete mess and made no sense at all; i tried playing it for 2 hours and then uninstalled it. MOO2 is one of the all time great making the next installment a must purchase. The reviewers couldn't understand what was going on either so they stcuk 'for die-hard fans of the origional' as a conclusion because they didn't want to be lambasted for missing another classic.
EDIT; just seen Oolon made the same mistake