Hi Rufus,
I beg to differ!
Plastics need razor sharp tools to work satisfactorily and they produce a lot of stringy swarf which makes seeing what is happening frustrating. Some types are also a pig to hold firmly, like trying to hold a lump of jelly!
For learning I would second the others, go and have a rummage through the off cut bins and find some free cutting BMS. Ali as mentioned can weld to the tool tip and then it just becomes impossible to cut well.
Instead of metal try machining some wood, say a few inches off a broom handle, it cost almost nothing and will give you a start before attempting metal. You won't get a good finish but you also won't break the tool if it digs in!