I might mention that as a side benefit of storing the reviews in a database, you get the possibility of adding value to your site by allowing searches of various kinds.
Besides the text of the review, and the rating, you could store the date/time it was created, the author, the IP address of the author (and the location, if you have an ip to location lookup service), etc., etc. And then you could offer searches on any of that data in addition to keyword searches within the reviews.
Text files are a simple solution to some problems, but they are so terribly terribly limited.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.