Light was okay, but I got used to his voice. He needs to work on his evil voice, but it's probably awkward to be the English dub of a kid with a god complex. I think as the series progresses he'll get better.
Ryuk...not great, but I loved when he said "Juicy." And his laugh was alright.
Lildemisa is right, we haven't heard L's or Misa's voice yet, so nothing's permanent right now. I actually think Light sounded young, but it's better than the trailer voice.
So, for now, I think Viz did a decent job.