Not a fan of on line check in. It doesn't guarantee your requests will be met, it doesn't even guarantee you will get the room type you booked (yep, had that lovely experience

). And if you are at all fussy about what location your room is in, if you will request a different one if you don't like what they assign you, it will take longer than normal check in.
I usually get better room locations not doing on line check in.