The South American dub is actually very good, and nowadays I always choose it when I want to watch an episode on my own, comfortable and relaxed.
However, it's not much of a problem for me to go from one to the other, I remember watching a few episodes of S5 in their original language at the time, and after getting used to watching mlp in another language after a few minutes, I didn't notice a big difference, because of how nice the dub is.
As this anon says >>7912
there are some translation problems. The most notorious and annoying for me is when they mix the terms "ponyhattan", "manhattan", "manehattan". Depending on the episode they use a different one, and it sucks.
And I'm still not sure if all those "20% cooler" jokes from RD in S6 were part of the original dialogue or changes made by her VA. In any case it's tiresome because of the repetition