It's a rare show that excels with both the SpFX and the science.
OK, to be fair, the science isn't perfect, but they really manage not to wave the 'poof, science is MAGIC' wand while still bending a couple of key principles to make the plot work.
The biggest SpFX of course is the smoothness with which the clones are on the screen at the same time. Add to this the skill with which the actress separates them one from another (Rachel imitating Sarah is just a little bit different from Sarah herself) and you've got a whole second layer of amazing.
I love the characters, the script is clever and funny and terrifying at the same time, and the science doesn't make me cringe. Love this show.
It has been years since I've read the book, but I've read it several times and loved it. The story has been massively updated in many ways both to make the technology and politics of the world match modern day, and to meet with the sensibilities of modern audiences. HOWEVER, the 'twist' at the end is perfectly true to form. For those reviewers who are complaining that we've seen it before, yes, because many script writers have read Clarke even if you haven't.
As far as why humanity tolerated the aliens coming in, read between the lines. Think about the powers the aliens have used onscreen. Just assume that those powers have also been used off screen. Direct violence has been tried. We've been shown one conspiracy which was tried. There was resistance, but it was useless. It's just been implied, not shown.
As for those who complain that the Overlord costume is a bit lame, well, it is. SyFy isn't known for it's budget. Having said that, would making the costume better have made a better story?