As Nick says, please ensure you use the search functionality of the site - you'll find a lot of your questions regarding OLCI answered with a lot of detail (and opinion!) over the last few months.
Is it worth doing? - It all depends.
Predominantly, it depends on what class of service you've booked and how keen you are to get the seat you want. Basically, I'd suggest that if you have the time (and it does only take a couple of minutes) it's worthwhile doing regardless. You know 99.5% that you've got the seat you want that way, and you have a little less stress to deal with the next day in case you're running that 20 minutes late or so. There are a couple of "quirks" however:
As Nick mentioned, the Fast Bag Drop area is getting more and more congested as more people use OLCI. I fear that this area *may* become even slower than the regular Economy check in area in time [:0]
If you want an exit row, I think these still need assigning by a check-in agent. This means you'll have to join the line regardless. Having said that, there's no harm in reserving your chosen seats online the day before, and then going to regular check-in to request any changes/exits.
If you're travelling PE/Upper, the line for dedicated check-in always seems a lot quicker than any of the "fast bag drop" stations I've seen. As above, however, there's nothing stopping you checking in online the day before and using the regular check in desk.
If you are going to the check-in desk, after having used OLCI - you can help the agent make things a little easier by telling them upfront that you're already checked in.
(Not sure exactly how, but I've had a tut or two when they "discover" I've already checked in online
[B)])
So, in summary - in Economy, it buys you some security, a little less stress and (for now) a little saved time at the airport. In PE or Upper, the time "saved" is really not that great (if any), but I certainly feel a little more secure knowing my butt's already supposed to be in my seat before I get to the airport
:D
