I use CO2, at least on one bike, but not because of weight. My 53 cm road bike with a sloping top tube doesn't have a convenient place to mount a pump -- not even something on the smaller side like a Topeak Mini Morph. (I've tried.) I keep a Genuine Innovations Air Chuck Elite CO2 inflator in that bike's saddle bag and it's served me well. One 16 gram cartridge fills one 23-25 mm tire reliably. On rides under 30 miles, I tend to carry two cartridges in case I get two flats or I botch an inflation -- which thankfully hasn't happened yet. More distance, add another cartridge.