I do online check-in because even if I still do have to wait in line, I don't have to wait as long, and there is a separate On-line Check-in line. I'll be honest, even when doing online check in, I've always ended up going to the front desk (unless staying CL which has a different check-in procedure anyway) because my room hasn't been ready (mostly my fault because I never seem to estimate my arrival time right).
Back in the days I requested bed rails, I usually called the "front desk" from the room and they were delivered then (It wouldn't surprise me if they never put them in your room for when you arrive, just incase of room changes).
The way I see it, if everything works out perfectly with online check in, I get to skip the front desk. If something is wrong, I'll have to go to the front desk (which I would've done anyway, had I not done online check in) so really, I'm willing to take the chance of skipping!