i have had the 51 for about 1 month now , and yes you can certainly fill the carts with a syringe , it's kind of tricky because you can easily flood the atomizer and your
juice will be un-vapeable ( whoa i think i just invented a word ) and therefore wasted. Also they don't sell blank carts so whatever flavor the cart originally was , there will be a slight lingering taste of that flavor. My first cart i filled was a menthol with clove flavored purejuice from puresmoker.com , interesting minty tobacco flavor , but mint and banana? no thanks
it's a touch overpriced , but if you are looking for CLOUDS of vapor then you can't go wrong , my 51 consistently produces more vapor than my 901 with a new battery and a new cart