Solar Panel Phone Charger

How effective are solar chargers for cell phones ? ?
I know it depends on the Solar Panel you get but overall how much time would you have to charge a phone to get it fully charged ?
How effective are they ?
Additional information is also appreciated.
It depends on the charger and of course the cell phone/battery in question. However, something like the Solio charger will give about 20-30 minutes talk time per hour of daylight its charged with.
If you are considering a charger, and it is imperative that you have charge for emergencies (why else would you have one) you should consider a hybrid charger; the solio is a hybrid and it means you can charge the internal battery from the mains as well as from the Solar Panels as required.