Crashes occasionally, but npapi crashed for me as well. Main advantage is that you get new flash version, that some games require to run.
Looks like Mozilla decided to ditch Flash, and now Firefox 40 sometimes hangs completely due to some odd interlocking with plugin-container process.
> Main advantage
... is that accelerated rendering is used where NPAPI Flash was afraid to use it. For example, for transparent plugin instances.
It comes with a price, though. There are more rendering issues, because unusual graphics paths are used.