Mitchie M did a nice job with English Miku in his Snow Miku 2014 piece. But of course he's a fucking genius at making Vocaloids sound realistic so it's not surprising.
https://www.youtube.com/watch?v=79N1O0lF0GY
She still sounds super weird though, like it's another person singing.
Also she's more rapping than singing, so it's probably easier.
I reckon the weirdness is in the way she says "snow skep" instead of "scape". I think if you just know to parry that stuff by typing in "skayp" "skeip" or something like that, you could eventually get her to sound more natural. Depending on how the software works, of course.
I once did like a radio show with voice synthesisers, and I sometimes had to spell things incorrectly to get the right effect. They were incredibly lifelike, but they'd sound odd on some specific things.
Here's a demo of that:
http://www.acapela-group.com/voices/demo/
One of these days I'll prolly get around to buying one of these vocaloids myself. Luka or Miku, English versions. Provided it plugs into Mixcraft, I'm set.
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Edit:
In related.. things! The MMD Mitchie links to is super cute! The MMD software can produce some really nice looking results when people use it right. :O
https://youtu.be/Yt7AoSS6tM4
That little train motion when she goes CHOO CHOO! Adorable.