Yes, polypins are fine for storing and dispensing beer. There would me no problem keeping beer in good condition for a good while, a month or two if you are careful with making sure everything is sterilised, and that no air gets back into the polypin when dispensing.. I prefer them to plastic barrels. There are various ways of using them. My preferred method is to fully ferment out a brew in the FV and then transfer it to a polypin with about seven heaped tsps of sugar. Leave with the tap uppermost, and make an airlock with a bit of tubing attached to the tap running into a milk bottle half full of water. When secondary fermentation dies down turn right way up and dispense as required, but keep an eye open balloning caused by excess pressure building up, and if necessary tilt the polypin and vent the excess pressure through the tap.
