Firstly, what siz tank is it for?
I've heard the in-line reactors are very good and in fact there's a DIY article somewhere on this forum but I can't find it. THey help keep the clutter out of the aquarium too. If you can't be bothered with DIY, there are some good one's on the market, the aquamedic one looks like's the bogs bollocks.
I've just invested in an Aqua medic reactor and although there are some design faults (the inlet pipe for the water input puts pressure on and kinks the CO2 pipe at certain angles) I've now got it setup and it's working great!!