Horizon is a prism... it is mostly scientific, retaining much of its earliest episodes in its latter ones (although not quite the details, especially when it concerns physics).
Its episodes concerning astronomy, in particular, are usually awe-inspiring, and although Horizon attempts to add in musical details that add to the whole picture (thus sensationalizing it, effectively) it does mostly succeed in conveying both the artistic aspect of what science could achieve and the actual details themselves, although when tackling topics that are more philosophical, like infinity, sensationalism may take over with not as many details.
Horizon also tries to explain sociological issues that verge on the scientific, like the placebo effect and autism; with the former it does succeed (in my opinion) in conveying both the hard science and psychology behind it, despite involving some random people... whereas, with autism, it fails scientifically, and seems to involve random people for no apparent reason.
Generally, as long as Horizon concentrates on the science it succeeds, but when it ends up speculating on anecdotes it doesn't so well (even with the earlier episodes, individual scientists were interviewed but they didn't necessarily talk about themselves).
Science should be at the centre of any documentaries that attempt to tackle such questions (autism clearly involves neurotransmitters e.g. and yet none were mentioned). Anecdotes should be used only in rare, specific cases like Henry Molaison's.
Overall, though, Horizon does seem to present science well...
