Just watched the first episode in Japanese and English. The animation definitely gets better as the series progresses. Subtitles can be used for English and Japanese, and they are the same, so it is easy to see where the English changes the dialog.
There are definitely changes. Some to remove crude dialog, others to make it more American. Although, at times, they are exact, word for word. The greatest dissapointment though is the Dragon Slave. The most important part of the spell is "I pledge myself to darkness". In fact, those words make a big difference at the end of Season 2. The entire point of a powerful spell is that the caster pledges themselves to some dark force. They don't just call on it, they let it invade their body. The English changed it. She swore to be a protector or something. Well, that did it for me. I definitely do not want to listen to it in English.
Besides, the voices just don't work anyway. Especially Gourry. The voice they use doesn't seem to fit his character.