Ooh one of the places I worked in New Zealand had a poo-picking vacuum which I used to pull with a quadbike. I could muck out 4 paddocks (2 horses per paddock) and the big field (4 horses) in around 2 hours, it was great! You can't use it on a steep hill as it will just topple over but slight hills were fine as long as I drove up and down the slope rather than along it. The toughest bit was emptying it onto the muck heap when it was full - did break a bit of a sweat shovelling it all out!
The poo hoover looks good. YO got one of the sweeper variety last year but found it only worked on very level ground and short grass. As most of the horses seem to poo in patches and the grass grows longer there because they don't eat where they poo, it turned out to be not so very useful and she sold it on.