The 100ml package comes in a presealed bag, makes it really convenient to use. If you buy the larger sizes, you have to provide a bag to put it in, but you must buy a bag that has very small holes, like 180 microns, the purigen granules are extremely small.
I have several of the 100ml packages, 1 in each filter on my 48gal tank, so I can recharge them at different times. They usually last about 2 months before they need to be recharged. The more you get, the longer it takes before you need to recharge it. So if you use the 100ml bag, you will need to recharge it in 2 months, but if you use say, 250, you can go for 5 months. (it all depends though, when you do your filter maintenance, look at the bag and see if it's all brown with no white left, when this happens, its time to recharge)
Recharging is very easy, just take it out, put it in a cup (must be a red solo cup :P) add a 50:50 sol of bleach and water, or just do straight up bleach, and let it soak for a day. Then rinse it in water and soak for another day in water mixed with Prime or any other de-chlorinator and you're good to go.
The only issue I've ever had with purigen, was forgetting about it while it was recharging.