A lot of people will.
You want them to roll so they will be IMMEDIATELY useful for a character who gets one: So, +xp, hell yeah! Random rolls give us a reason to try and get a better one.
But +primary stat? That SUCKS. if 200 primary is say, on average, 2k dps for a dude, figure out the equivelant in +damage, or crit/crit dam, or ANY OTHER STRAIGHT DAMAGE FOR EVERYONE STAT. SERIOUSLY.
Do NOT set these rings up to be SPECIFIC to a class, because that is obnoxious and will frustrate anyone who gets a bad roll.
What I want to see is something that by default, is useful to ANYONE, and everyone! And then can roll more specifics from it's pool of random stats.
I spent some time thinking about this, and all I can think is that it will frustrate players, pretty rapidly. I feel like a few bad rolls will make the event feel terrible, even if you can farm it rapidly.
I think this should have completely game-breaking feeling stats. Like, close to over the top insane, by itself: it's one slot. And clearly, and event ring like this is meant to be end game sort of stuff. So please, don't make it a decent ring, make it a godly ring, and make it useful to anyone who touches it for damage, or for defense. Or hell, have a defense and damage roll version!