They're correct that evolution is not repeatable in the sense that we can watch humans evolve from fish again, but it is observable. Check out Richard Lenski's Long Term Evolution Experiment for a great example of this.
And Darwin was concerned about the lack of transitional fossils in the geological record, but that was well over a century ago. Since then, hundreds have been discovered, including transitional forms between fish and reptiles, reptiles and birds, and apes and humans.
This page goes into fascinating detail about the evolution of whales from land-dwelling quadrupeds, with half a dozen transitional forms described. This evidence alone is utterly convincing, and it's just a fraction of what we have.
It would be nice if creationists could present just a single piece of evidence for their own theories beyond a 2000 year old book.
More laughably inept creationist propaganda from "Tangle" in future posts.