If it were me, I would wait until the day of to buy. To book ahead might lock you into a day of fog, rain or other issue with visability, making it a less than ideal outing.

My daughter and I just turned up at around 4 pm on a Wednesday afternoon and waited less than 5 minutes for tickets.

Ditto what Kansasbeck said. I'd suggest waiting at least until you are in London to purchase tickets; perhaps you can check local weather forecasts. We ended up going on the Eye by happenstance as we were exploring London early in our stay. The weather, though not the best, was relatively good at the time, and we waited not more than 15 minutes to buy our tickets and board after just walking up to the Eye with no reservations. It was absolutely wonderful, and I would not have wanted to miss it.

The London Eye website gives the impression that you can buy same day tickets online and receive a 10% discount. I made a dummy same day booking up to the point that my credit card number was required. Regular tickets were available about 4 hours out.

The fast track and other special tickets require you to bring an e-mail confirmation. I would not want to spend my London time online waiting for an e-mail confirmation or trying to print it at an internet cafe.

I bought my tickets on-line and received a 10% discount. I only had to produce the credit card I used to pick up the tickets from a machine--didn't have to print anything to bring with me, and didn't have to speak with anyone.

One thing--even though my ticets were for 7:30, and I was early, they were ready to let me in at 7--any time, really, as the London Eye never stops and they want to keep everyone moving.

I should also mention that we went in person--the lines were huge, so we revised our plans, booked on the internet, and went back the next evening.

There's a nice Marriott around the corner for a quiet drink to get Far from the Madding Crowd.

I totally recommend buying your tickets online in advance but not too advance. You want the weather to be nice and since you need a reserved time, you should make sure you are free then. Things like these are reasons I pay for the internet in the hotel.

As others said, booking online gives you a discount and the line to pick up tickets is MUCH smaller (think 1-5 people vs 30-50).

I did the eye last year from our home near York and found booking in advance a great help. I understand the concerns over weather etc but if coming during a busy time the advance booking save standing in a queue for the tickets. Advance booking means you collect your tickets then stand in a shorter queue for the ride itself. I also found that booking s river cruise at the same time but to be taken before the ride on the eye gave you an understanding of what you were seeing went you were up in the eye capsule. March should be OK either way.