People who've seen this series don't need to be told whether it's good or not, so this review is solely for the uninitiated, i.e. someone trying to decide whether to invest the time in a seven season series. Here's what to consider.
This was rated, when originally aired, as TV-14 programming, but it is far from your typical YA drivel. It covered a full spectrum of emotional, violent, and sexual content, over its seven season arc. Ignoring the sex, and just considering the human (non-"creature") body count, I'm amazed at what the showrunners were able to get aired over the course of the series. Bottom line, there's a lot of "intense" content throughout the series, and it gets far more graphic in the latter seasons.
Any fantasy, supernatural, or SciFi series fails or succeeds on its "believability". I don't mean the plots, but instead, whether the characters project believability. If it "works", it will stem from a combination of well scripted episodes, but far more important, a cast that buys into, and fully "sells" (110%), their characters. BtVS had all of that in spades. Whether it is their witty banter, a plethora of deeply emotional scenes, or the larger story arcs of the seasons, and/or the overall series, as a whole, these characters are "all in", in terms of their commitment to the stories being told, within their "fantastical universe".
Throughout the series, there are always multiple (concurrent) story arcs in play. Some, especially toward the end, are even multi-seasonal. Certainly, simply because it is episodic, there is a "monster (or problem) of the week" feel, all through the series, but those stories are generally unique and interesting, and many of those play into the larger (and more important) story arcs.
In the first three seasons, the characters are in high school, and many of the plots revolve around that setting (but always with the fantasy-based twist), while the final four seasons move out into the adult world. We basically get to watch these characters grow up throughout the full series arc, and as alluded to earlier, the plots become increasingly more "raw/intense" as the series moves forward.
The most important thing is that, even from the first season, it is easy to become vested in the core suite of characters. And, when season two takes what seems to be a somewhat stable set of core characters, and turns their situation "on its head", you realize that you cannot count on anything being "stable", and consequently, can't really predict what might happen in subsequent episodes (which, obviously, is a "good thing", from a story-telling perspective).
Some of the later (especially season 5 and 6) story arcs are heart-wrenching, and easily some of the best "TV drama" that has ever been aired. Some of the episodes are also some of the creepiest that have ever been aired. The series is a constant mix of many different storytelling genres, which just happen to exist in the (fantasy-based) "Buffyverse". Unlike some series that fizzled out near their end (or simply got canceled), this series really reached its peak in its final three seasons, just continuously "upping the ante", until its epic finale. I own the series, and (roughly) every two or three years, I drag out the discs, and re-binge the whole thing, because of "Buffy withdrawal". It's always feels good to (yet again) watch it all play out, over the characters' seven year arc.
BtVS is "not perfect". There are some episodes that are (IMHO) duds ("Beer" comes to mind), and there is at least one story arc that I thought had a bit too much silliness, but "as a whole", it is easily one of the best TV series, and runs you through the full emotional gamut.
Bottom line, assuming that you "accept" a fantasy/SciFi premise in the first place (as you would with a "Star Trek", a "Fringe", or any similar series), then BtVS is truly a great series (for people of any age... and I'm "North of 60", as I write this), and I believe, well worth your time investment.
One major caution: The first two episodes of season 5 are a major "WTF moment". During its original airing, some viewers (literally) stopped watching the series after seeing those episodes, and also wrote online rants about their disappointment with what happens (avoid reading the episode synopses to avoid spoilers). Those episodes definitely DO leave you asking "WTF", but I would simply say that you MUST have faith, and "trust your tour guide" (i.e. the screenwriters), and settle in for the season 5 "ride", which, as I alluded to above, is an emotional roller coaster, with an "unexpected" season finale that sets up a similarly emotional season 6.
One final note, depending on where you obtain media, and/or from where you stream it, seasons 4-7 may or may not be wide-screen. The latter seasons were shot on 35mm, whereas the first three were shot on 16mm. Even though the entire series aired in the old (narrow) 1:1.33 (original NTSC TV) aspect ratio, and even though Joss Whedon argues that the whole series should be seen that way, I respectfully disagree. I always relish getting to Season 4, where (with my UK DVD versions) the whole series opens up, and fills up the full 16x9 (modern) wide screen. Scenes, and "people", that would have been partially "cropped" (or shifted) in the narrow format, are simply seen "in full".
So, hopefully some of the above is useful, if you are considering the series. It is definitely a unique experience.
