Charles Darwin’s theory of descent with modification by natural selection is one of the truly great ideas of humankind has produced. And while he got some things wrong and 150 years of further research continues to modify and refine his theory, his basic premise that all living creatures gradually change over long periods of time still holds true. 
That Darwin got things wrong is no big deal. He was (one of) the first to describe the theory of evolution, which would by analogy mean that he was also the one who knew least of evolution. He didn’t know about DNA or transitional fossils between fish and amphibians, and his understanding of genetics was messy. But every scientist since Darwin has been able to use his work as starting point, fill in the blanks, and advance it even further.
The Theory of Evolution, as it stands today, is a fact. It seems to me that the most common objections are based on ignorance of evolution and scientific principles in general. Saying that it’s just a theory is nothing but silly wordplay. Humans did not evolve from apes, apes and humans have a common ancestor, and took their own evolutionary way from there. Likewise, people who claim that evolution isn’t true because no cat has ever evolved into a dog just prove that they lack even a basic understanding of evolution. “Survival of the fittest” does not mean that only the strong (should) survive, but that those organisms that are adapted best to their environment have the best chance to produce offspring. (Also, it wasn’t Darwin who coined that phrase.)
As for The Origin of Species itself: getting through it required a lot of attention and determination. I’m not too big on non-fiction in general, and some evenings I had to struggle to keep focused on the text. That has nothing to do with Darwin’s writing, which is very clear, and everything with my attention span, which resembles that of a dead goldfish. Following along with John Whitfield’s Blogging the Origin series proved to be a good decision, as it forced me into a realistic schedule, and it provided feedback on what I just read. For example, when I thought that chapter 8 on hybridism was, to put it mildly, a bit tedious, he more or less confirmed what I already thought:
Unless you’ve got a jones for Victorian horticulture, you could skip this one and miss nothing. Reading it is like listening to someone describe, one point at a time, an extremely large scatter plot that shows no correlation. So species rarely hybridize. Except when they do. The offspring of such hybrid matings are rarely fertile. Except when they are. While I would have made my way through The Origin on my own eventually, it would have been a lot less fun.
So while it was at times a struggle, in the end, I think it was definitely worth the effort. When Darwin is on to something, he’s very persuasive in making his point. He raises difficulties on his theory, only to come with an elegant explanation later on. To give one popular example, let’s look at what Darwin says about complex organs, such the eye:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
This quote is often used by evolutionary nay-sayers as proof that even Darwin thought that the eye could not have evolved by natural selection. But they (deliberately) leave out the part that follows, in which Darwin he refutes this criticism,
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
And then he delves deeper into how, over a long period of time, the eye could have formed in a series of small steps from a primitive, light-sensitive cell to the human eye, while documenting examples where the steps in between can be found in many animals.
As a final note, it could be interesting to note that if you would search the text of The Origin for the word “evolution”, you’d get zero results. The closest Darwin comes is in the very last sentence,
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
- Charles Darwin — On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.
- I must restate that the last time I encountered a scientific text on biology and evolution was over ten years ago, and I’m by no means an expert on this stuff. So if I mess this up, blame me.