Milk bottles are the usual target that people start with when they first pick up a sword and theres a reason for that, Theyre notoriously easy to cut. If you can't cut a milk bottle with your sword then something is seriously wrong and that makes them an excellent first target to have a go with. They're a good confidence booster because you may feel awkward about swinging a 3 foot long piece of sharpened metal around in the beginnning and theres nothing wrong with that. It takes a while to get the feel for it and in a perfect world, you want to be rewarded for carefully thought out cuts that happen easily with no need for overswing and no chance of the notorious 'bottle bounce' that you sometimes get with poor hasuji.
It's also this reason that a lot of people decide to leave milk bottles and move onto the standard 500ml soft drinks or mineral water varieties as they improve. They provide a smaller target area, theyre more difficult to cut and they make a fantastic 'snap' noise when you get the cut right. But that's not to say that milk bottles are to be forgotten, once you've gotten to this stage. They still have an advantage even to a veteran cutter and that is to train in accuracy. I've seen a lot of videos (I have some somewhere) where a couple of cuts are done on a milk bottle and then it's thrown away. There's so much of the bottle that remains uncut and the reason it's left this way is simply because the person cutting didnt try to make as many cuts as possible to the bottle. Try taking the smallest slithers of milk bottle off at a time, because they cut easily and therefore the only thing you have to concern yourself with is your aim.
Sometimes groups of enthusiasts, usually online, have 'milk ring' competitions where you have to get as many complete i.e. not broken or partial, ring slices from a milk bottle as possible. It means that you'll have to make the slices as close together as possible of course but the closer you try to make them, the more difficult this becomes. It's a good way to get your cuts to be nice and tightly grouped and because you can line up cut after cut without having to put another target up on the post, it's easier to adjust your cuts straight away.
So, in short, if you've decided to leave these milk jugs because you don't see the merit in cutting them, start saving them again because theres still plenty to practice on. Whats your record number of complete rings on a 2 litre milk bottle?