They work great, much more efficient than using an inverter. You need at least 2.1 amps to charge an iPad at full speed. You should look for 4.2A (or higher).

I've had several brands and so far no reliability issues other than a metal tip that unscrewed and got lost. The link below is the latest one I ordered. (Only been used for a few days so I can't review it other than to say it works fine so far. BTW it came boxed with a tablet pen and screen cleaning cloth, not expected. Sweet deal.).