From all plugins most used are ones for color grading and titling. I am sure that they'll be available soon. Why we don't see them? Simple - Apple did the right thing and not allowed information leaks to competition. FCP is one of few key applications for Mac platform, so they couldn't allow situation with new Premier having same interface at almost the same time.
Cool stuff. Supposedly no gamma shift during transcoding to ProRes. Built-in image stabilization. Better color correction. Yeah audio sync. Dual system recording!!!
But it seems lacking integration w/ other tools such as AE CS5. No third party plug-in... yet. If you don't use anything other than FCPX, no brainer getting it asap. Frankly Apple just killed 3rd party providers... right?
Otherwise FCP 7 + PP PS AE CS5 provide good workflow. Trascoding & quick editing from FCP 7. Export XML. Import from PP. PP being a channel to AE and PS.