This is an excellent device. I have three water butts, and installed three different diverters over a few months (I bought the cheapest each time!). This was the easiest one to fit - it's always a bit fiddly getting the level right! - the best looking, and the only one that was entirely trouble-free.
Now buying another to replace the cheapest one which has always leaked a little through a poorly moulded connection.
Not the cheapest, but the best, and only a quid or two more. It can be adapted for square down pipes, but you'd do better to buy the square-looking one.