I did it for my flights to the UK over the xmas hols and for the 1600 miles I managed to clock up in the hire car. I don't think that I should have had to do either as it should be included automatically. Happily, any airlines flying into or out of the EU now have to pay the carbon tax if they exceed their (tradeable) limits. It's a pity these carbon credits were given away rather than sold to begin with and not sold but it is a good start.
I am really happy the EU resisted pressure from airlines and other countries not to introduce the tax. See, for example, this stupid article from the Washington Examiner which thinks that the EU is trying to get foreigners to bail out the failing European economy - um... Europeans have to pay the tax too; increasing transport costs hurt not benefit the economy in the short run; and, quite frankly, we will all destroy all of our lives if we done implement schemes such as this - in economic models, that results in a utility of minus infinity making this excellent long-run economics.
Carbon offsetting the flight and the car journey makes them both more expensive but if I don't pay, someone else (who is probably significantly poorer than me) will pay for it.
I use myclimate.org to do my carbon offsetting.