What makes .mac worth the $99/year to me is the homepages. I'm not skilled enough to create a webpage by myself, and the free ones out there are not very good. With the homepage, I can easily post all my pictures, and they actually look very nice and sleek. I've had a bunch of friends ask me how I made such a nice homepage. They have no idea that I just pick a theme, upload, type some descriptions and arrange! It's also nice to be able to upload movies and have pages of text.
The ability to share files is also nice, but it may only be useful for certain situations. For example, my biology teacher gave us his homepage address at the beginning of the year, and we were able to download documents, notes, presentations, assignments, quizzes, lab pictures, etc. from his iDisk. For him, it was worth $99 to be able to so easily share lots of files with all his students.
I don't use my .mac email address, I've had my gmail address for a while, and I hate switching addresses. However, I did add my gmail account to Mail, not sure if you can do this without .mac or not.
The freebies are nice, but no big deal as some other posters have said.
I think basically what it boils down to is what you're going to use it for. Hopefully the free trial will help you decide whether you need it or not.