It's fairly good, but it has some big issues:

(0) *** It needs to allow per-site overrides. You should be able to set a global user-agent string, but override it with a different one for specific sites. ***

(1) It doesn't get regular updates to keep the user-agent string up-to-date with new browser versions.

(2) If you modify the string of a combination of OS+browser, there is no way to reset it to the default, and the change will be retained even if uninstalling and reinstalling.

(3) No option to save a custom string; if you change the agent, the custom string is lost.

(4) No option for random agent.