turns off "most" logging. ( Fine points escape me, but likely someone else will have these on the tip of their tongue)

speeds up the database, sometimes significantly.

makes database UTTERLY USELESS if you have to recover in ANY WAY whatsoever ( including simple instance recovery ) while instance is in this mode.

If you do it, can do a shutdown NORMAL, remove the parameter, and restart, you've survived.

I'll assume you're an adult like everybody else on the list and leave the threats, warnings, and remonstrations to somebody else. You've got enough information to decide for yourself whether or not to use it.

Last time I looked into it (7.1.6-ish) redo generation took place as normal but the call to write to the file was bypassed.  So CPU still used as normal, but no waits for I/O.

This did speed things up a bit, but not dramatically; checkpoints still occur, by the way, and they can have a bigger I/O impact than that redo writes.

No fine points about it.  It turns off redo logging TOTALLY.  As with all the underscore parameters, it should never be used without Oracle Support telling you to use it.  This one is particularly dangerous, as you mention.  Any failure is simply not recoverable from, so it should never be used in a Production database.