I've tried JG a few times, and must sat I think they're one "gogo good good" fitting easy, and always if you make get any mistakes are always easy reassembled. Plus the one thing I must rememberd here is always use a pipe support fitting for the pipe being used. So if you were to be using H2o pipe and a JG coupling, use the recommended steel H2o pipe insert.
Dont even think about using a JG fitting with a Hep pipe - thins is the one no no!
Use a compression fitting to joint the 2, as that is universal but use the correct inserts, as per the manufacturers instructions!