Inno Setup provides 90% features you need to create a standard and powerful installer while 10% features you may want are missing. For me, I hate to hard code version strings in the script and update it several times.
And now I find Inno Setup Newsgroup and Inno Setup Extensions Wiki, so most problems can be solved following the tips.
ISPP is mentioned so many times by IS experts which means it is a "must have" tool. Yes, after reading about it I find most missing features of IS in this useful preprocessor.
Inno Setup can serve most people while only a few of them learn and use ISPP. For example, I've been using IS for a long time and finally decide to learn ISPP today.