FCPX has been a great editing tool but one of the things that always bothered me was the lack of simplified exposure, color, and sharpening tools. You have to jump through so many tabs as opposed to just scrolling down with everything in one place. Today I decided to test out a very very inexpensive plugin called HyColour Pro and I have to say it's an extremely helpful plugin I think everyone should consider. Once installed you simply drop it to your video clip, and the tools show up in the Video inspector.
It's extremely intuitive and feels much like Adobe Lightroom with basic sliders that control individual components to quickly correct or grade your footage. This plugin can save you an enormous amount of time in those areas. I think the one feature I favor the most is the comparison overlay which allows you to do side by side or split screen comparisons to your original footage.
Unfortunately you'll run into problems with projects larger than 1080p, but hopefully they'll be working on an update. If you're just starting to use FCPX, and are still focused mainly on 1080p content, then you should definitely check out HyColour Pro (click here).
HyColour Pro Color Grading Tool for FCPX