I highly recommend visiting Mutianyu. We went in winter and it was snowing. The views are stunning and the climb provides excellent views. My sister has also visited Badaling and said Mutianyu was far better to visit because Badaling is overcrowded with tourists and has lost its authenticity due to reconstruction of the wall to make it easy for tourists to climb.

At Mutianyu, we only shared the wall with a handful of tourists. We considered visiting Simatai or Jinshanling but we were told Simatai's steps are far steeper and harder of climb (not great if you're afraid of heights) and Jinshanling is much further away.

In terms of how to organise, we did it through our hotel (Park Plaza Beijing Wangfujing) at a cost of 420RMB per person. It included transport in a car with a driver, an English-speaking guide and lunch.

Having visited both entrances accessible to the Great Wall from Beijing, Mutianyu is the only way to go! The nice touristy entrance (whose name I can't remember) was just that, nice, but several hundred steps (of varying depths) before you get onto the wall is not my idea of a pleasant experience. Mutianyu on the other hand has a chair lift to take you close to the wall, then about 6 steps and you are actually on the wall. We were fortunate enough to ride up in the cable car in which Bill Clinton rode when he visited China as President. There is plenty of wall upon which to walk, plenty of stairs if that's what you want to experience and certainly plenty of "goodies" to buy on "ground level". I would highly recommend a trip to Mutianyu and you could also visit the Ming Tombs which are in that general area.

Mutianyu is beautiful. We were there December 23, 2007. We also went to Badaling. Mutianyu is superior in every way. I'm happy that we did both, but if you can only do one, my advice is to go to Mutianyu. We rode the cable car up and back and went to the left, to the end of the restored section, when we got up to the Wall. There were NO vendors on the Wall at Mutianyu in contrast to the many, many very, very pushy vendors on the Wall at Badaling.

We took a tour with Beijing Xinhua International Tours. It was a great visit to the wall at a really quiet spot, but it was riddled with shopping I wanted no part of.
We were taken to a Jade factory first and given a tour of that and left in a showroom where you feel obligated to buy something, then taken to a enamelware manufacturer where you are again given a tour and dropped off at the showroom expected to buy something, ten taken to the wall finally, then picked up in 2 hours and taken to a nice lunch place, then taken to a silk factory where you are given a tour and you guessed it, dropped off at another showroom where you are expected to buy something.
The tour said we would be back at about 1:00 Pm and we didn't really get back till about 5 PM, so be careful about planning things later in your day.
You're probably better off getting yourself there and back, cabs are ridiculously cheap and they don't tip, so just do that if you don't want a little wall and alot of shopping!