<p>I've never used CI tools before, but from what I've read, I'm not sure it would provide any benefit to a solo developer that isn't writing code every day.</p>

<p>First - what benefits does CI provide to any project?</p>

<p>Second - who should use CI? Does it benefit all developers?</p>

The benefit of CI lies in the ability to discover early when a check in has broken the build. You can also run your suite of automated tests against the build, as well as run any kind of tools to give you metrics and such.
Obviously, this is very valuable when you have a team of commiters, not all of whom are diligent to check for breaking changes. As a solo developer, it is not quite as valuable. Presumably, you run your unit tests, and even maybe integration tests.  However, I have seen a number of occasions where the developer forgets to checkin a file out of a set.
The CI build can also be thought of as your "release" build.  The environment should be stable, and unaffected by whatever development gizmo you just add to your machine.  It should allow you to always reproduce a build.
This can be valuable if you add a new dependency to your project, and forget to setup the release build environment to take that into account.