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!

After looking at reviews from Tripadvisor decided this would be the best part of the wall to see. Arranged a regular taxi to take us for 700RMB (could've haggled it down to maybe 400 we found out later but didn't as thought it was fair price anyway). Taxi took just under 2 hours from the centre of Beijing and the driver then showed us where to get tickets, etc.
Took ski-lift up and tobaggan down - total cost including rides up and down, wall entry and compulsory insurance was 101RMB.
If I went back again I would take the cable car up then walk along the wall to the tobaggan down as there is more downhill this way and that route would form a natural triangle.
This is an interesting section of the wall as it's so varied with steep climbs and then lesser ups and downs. There's very little flat so if you're not prepared for it you won't get very far.
We spent nearly 4 hours in total on the wall walking along the wall in various directions. If we had gone on a tour it would have cost 600RMB for 2 people and we would only have had 2 hours actually on the wall so very glad we took the taxi and had the freedom to spend as long as we wanted.

Not only is this the best part of the wall to visit, it has the greatest decent of all the other sites. As you have heard from other reviews, Badaling is not the place to go. They are building 5 star hotels near Badaling so that should give you an idea of how crowded that section of the wall is. Mutianyu provides great "people-free" photo opportunities. I can't say enough about this section of the wall.

The best way to see Mutianyu is to take the closed cable cars to the top, once you reach the top, take the left route, you will be able to make the steep climb to the highest point on the wall, but if you are not so adventurous, then you can walk left for some great photos without climbing the steep wall section, work your way back past the cable cars and walk all the way to the ski lift / tobogan area (this walk is mostly downhill), TAKE the Toboggan down back to the parking lot area where you began. This is such a great way to end your hike, Don't worry, you can control the speed of your descent!!!

If you go to the top via ski lift, just be aware that your walk will be about 80% uphill, everyone I passed walking this way were disappointed to know that the other route was better.