I have a D whistle that's made of plastic, and it's a great whistle. As the article mentions, it doesn't change pitch as it's played, although, I can move the mouthpiece to change the pitch slightly.
The nice thing about plastic whistles is they're durable, but mine is a good one and it was about $35, if I recall correctly. So, that's a consideration, since there are so many whistles on the market that are much cheaper, and still sound really nice.
Some websites that specialize in Irish instruments have sound samples so a buyer can hear what a whistle sounds like before buying one. I like a sweet, mellow tone and a whistle that "flips" into its high register fairly easily. I also want a mellower sound even in the high registers, since sometimes they're a little shrill.