I think it depends if you can find a good match for your horse and approved by your trainer it's a good way of saving money and get horse stay exercised and fit. However, there is also risk of getting horse not ridden properly, especially for green and young horses in training. If anything bad happened it is hard to argue who should be responsible for it.
I think It's a good idea but you need to make a contract to avoid problems. Sometimes, there is an accident and the question remains outstanding : "Who is responsible in case of accident?" the contract is here to avoid this kind of question. In which you define clearly all the aspects to your half-board. Who pay what (for example dentist or veterinary,...), the distribution of days, how use the material placed at its disposal. And what the half board can do !
You can find half-board in your club or in different websites such as federation, association, breeding farm, ...
Half boarding can be a great option !
If you are looking to save money and get your horse more time under saddle and is also an ideal option for those who are looking for riding time but are not ready to own a horse !
However, it's important to take a few legal aspects into consideration ...