Does anyone know if the optional study guide for Bio 1M03 is useful? Is it worth buying?

It depends on your studying style.
Personally, I didn't buy it and did fine with the class notes & textbook only.

I found it really useful. It summarized the textbook, so I only read the study guide

The guide also includes a series of multiple choice questions at the end of every section which were sometimes also used on the midterms/exam

I found the guide to be much more useful than the textbook, it has section summaries and practice questions, and outlines all the main ideas for you, whereas the textbook is very long and readings can get tedious

Also, the study guide has both 1a03 and 1m03, so you can use it for both courses definitely useful!