I would like to know if there is some plugin for this kind of effect: fast slow motion fast. For example you can see often in GoPro action videos that kind of nice smooth clips. I know I can blade the clip and then fast or slow each parts. But I would like to know if there is somewhere plugin which make it to do faster and more smooth. I use Final Cut X.

No plugin needed, just use the Blade Speed tool, and set the speeds inside FCPX itself. Look under the "Create custom variable speed changes" section to get started.
help.apple.com/finalcutpro/mac/10.1/#ver95783cbbc

And here for more info:
www.macprovideo.com/hub/final-cut/fcp-x-...oring-retiming-tools

Thanks! Yes, Final Cut X has that tool. My Final Cut X version was about 1 year old. I had problems with that tool. When I used that and then set Video Quality Optical slow, some errors appeared on that clip. The red line did not dissapear. When I send that footage to Compressor, I could not succeed to convert any file. When I took that clip away, there was no problem anymore. I think Apple had bug on Final Cut X.

But yesterday I updated Maverick in my computer and now I have lates version of FCX. There is no problems anymore. So this is tip, if somebody has similar problems. Now I can use that tool well.