The only valid complaints about co2 are
(1) it's temperature sensitive.
That's not a big deal if you shoot at a resonably pace and keep the rifle's temp. stable. Day to day temp. canges can screw your zero. Still, most days it doesn't change temperature by 20 or 30degrees, so its more a seasonal problem than a day-to-day problem.
(2) It costs more.
Using the best local prices, am paying about $.032 per gram for 12gr. powerlets ( $15.43 for a box of 40 cartridges). That works out to $14.00 for the gas to fire 1000 pellets.
Bulk fill costs a good bit to set up (tanks. adators, hoses, etc.) but the local proce is running about $.006 per gram (and that depends on the shop's urge to rape and pillage and how much you waste goofing off squirting things). That works out to $6 of gas to fire 1000 pellets.
So if you intend on shooting a whole bunch, saving $8 per 1000 pellts will pay for the bulk fill equipment mighty fast.
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With that said, I still haven't bulked any of mine. Generally, my shooting is more about a few shots a session, with long breaks between each gun gets an outing, so cartridges do well for me. My day-to-day shooting (and I do shoot a little EVERY day) gets doen by two very inexpensive air rifles:

Yep...the dreaded "D's"...but considering my back yard range is only 25yards, and the only live targets at home are pest birds, they work fine for every day practice.