How can I reuse an effect sequence in FCPX?
Use "Total Adjustment" it works like a photshop-layer. You simply can duplicate it for reuse with other clips.

How to: Install Total Adjustment. Open the effect-library in Final Cut Pro X. Select the titles. Drag the "Total Adjustment"-title to your project. Copy all effects you need to this title-clip. Place it above a new videoclip. Every video-clip that's under this kind of "layer"-clip will be affected of its effects and adjustments.