My original files are 1080 P H.264 at about 28 Mbps. When they go in a project timeline, FCPX renders them to high quality files which are roughly 5 times as long. This is relatively transparent in the background and the editing goes very well this way. My LaCie Raid 0 drive as the speed and space for this, but of course at the end there are huge render files, which can be discarded so most of the file bloat disappears. Is there an advantage in editing with the high quality files (rendered) over using the original H.264 files? Typically I am editing an hours worth down to a 5 minutes film, so in the end, if the render files are trashed, it can be re-rendered quite quickly.
FCPX is rendering in ProRes. If your source files are H264, then you don't really gain anything by rendering in ProRes except for some optimization in playback. But I haven't had that much trouble playing back H264 in editing so I probably wouldn't bother rendering until the end (for export).