I can't remember who it was, but someone on here recommended E-Lites to me.
I'm using them now. They are not as flavoursome as some of the other brands but they are 100 times more reliable. I've tried loads and had problems with glops of chemicals falling on your tongue, batteries that fail in weeks, cartridges that don't work etc. So far, with E-Lites, out of hundreds of cartridges, we've had only one faulty one, and they added two free ones to our next order.
This is the kit we use:
http://www.e-lites.co.uk/epro4.html
As you can charge the carry case as well as the separate batteries, if you are out and a battery dies, you can switch to the second battery while recharging the dead one in the case.