I've experimented with portable solar panels in the past for charging small electronics, to not much success. These panels are the first lightweight, durable and reliable solution I've found for quickly charging batteries, and I used them basically every day the sun was out to charge my Nexus, flipcam, GoPro, as well as an external battery pack which I used as a backup during the night and overcast days.
Covered in a polyster canvas with built-in velcro and metal grommets, the panels fold up into a very thin form factor, and when unfolded are very versatile to hang or strap down. Using a couple small bungee cords I was able to strap the panels to one side or the other of my panniers (depending on the time of day and route direction). This held the panels in place tightly, and with great angles for catching the sun.
I will be bringing these panels with whenever I travel outdoors - they are very reliable, and with 14W and two USB ports, it's easy to keep a couple devices or a battery pack charged for later use.