I have booked on MU both times in the UK using GotoGate, The booking process is fine (chose seats, etc), but I cannot comment on their service in the case or IRROPS as so far, not needed them.

That said, both times I booked the MU website refused to take payment and I got the endless wheel of death. So I had no choice. And given the poor translations and clunky website at MU, I think you are better off not bothering and going straight for a third party, something I would not normally recommend.

One problem with MU is that for some reason they won't allow you to acess the booking on the website once it's made with a third party, so make sure you are happy with the seats you choose before paying. I could not do an online checkin, but there was no issue at all at the airport and armed with the ticket number and PNR you should be fine.