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Never used the bag in a box style ones, I believe the are not as strong.
Serving by gravity is the most straightforward. I fill the pin direct from the fermenter. You can expel the air by squeezing the pin, this lessens the chances of oxidising the beer during transfer. One tube will connect most FV taps to the polypin. I like to give the beer at least 3 weeks conditioning although I have drank after 10 days and still got a decent pint. Generally I would prime about a week before drinking, the short time the tap was open has never had any noticeable effect on the beer. Prime with 60-80 gms of sugar, rock the pin a bit to disperse and move somewhere warm. Once the pin starts to expand move to a cooler serving place and leave at least 2-3 days to settle. In this time the pin should expand fully. Sometimes it can look like it is about to pop. I have had them as hard as a football before without a problem. If you are worried you can roll it back carefully and release some gas. I have done this myself and managed not to disturb the sediment. I use a 20 lt pin, although normally get 23 lts in, and found it to keep fresh and in perfect condition for at least 2-3 weeks. Even known it to continue to produce gas right to the end and remain inflated like this.

This was after 2 weeks and the last pint still had a lot of life.

If you are using this method then you will need to experiment to find what works for you.
Been serving my beer via hand pump for the last 4 years now and have to say it is worth the extra expense if you want a beer as served by hand pump in a pub.

When using a hand pump I have stopped adding priming sugar as I found the action of the hand pump gives my beer sufficient life and doesn't need any extra priming.
With an Angram pump you can connect direct from pin to pump, I just use a jubilee clip to secure.

I use a food grade silicone tube and rinse the whole system with beer line cleaner between pins and clean the pins with oxy cleaner.
If treated right they can last years although I tend to replace after a few years.
I don't use the cardboard box and never had any problem with skunking but there is no direct sun light in my garage.