The vast majority of soalr panels are ultra-violet (UV) light powered. UV penetrates cloud but is also attenuated by that cloud, or rain. So, solar panels will charge in cloudy weather but not as quick.
The issue, actually, is whether your solar panel is outside, or inside. What do I mean by this? Well, most glass is manufactured with a UV filtration effect - ie, it blocks UV to reduce glare and the sunburn effect. This also means that a portable solar charger will take 5-6 times as long to charge if it is behind a window, even on sunny days.
So, the question is now actually, can I put my solar charging panel outside?
I've been using the Freeloader with a Supercharger external panel (called the 'Globetrotter' system when sold together), which is weatherproof, and it works well. Very Ronseal - the charge timings are just as it says in the manual. However, even with the bigger panel, it still takes much, much longer to charge if it is behind a window.
Freeloader won't charge anything with a battery as big as a laptop's but it will charge my Blackberry, my iPod, and my PSP. I also have the additional battery charger which means I can charge AA or AAA batteries too.
I really like this system but, as I haven't used any of the other makes (like Powermonkey), I can't comment on how it stacks up against the competition.