I came on to the Buffy bandwagon later in the series run. The movie came out while I was in high school and I remember it being pretty bad, so I did not really have much interest when I heard that a TV show was being made. As a result I never watched one episode until the very end of the 4th season and then from the 5th season on. I just happened to watch that episode as a fluke because the WB happened to be the only channel I could get on my TV right after I moved and before I got Satellite TV hooked up. Needless to say, I am glad that I did.
Honestly, had I started watching at the very beginning I may not have stuck with the show. The first season (12 episodes) was really about the show getting a foothold, and establishing the characters. As a result, the stories did not always flow as well as they did in the later seasons. What the show did do well, even from the first episode on, was take the experiences that most of us had in high school and as teenagers in the 1980s or 1990s and present them as real monsters and demons. When you look back at it, the show tackled issues like living up to parents' expectations, peer pressure, abuse, rape, death, and sexuality (to name just a few) at a time when I was not politically correct to do so. The show, at its core, is really about growing up.
I personally think the show peaked with the 5th season. The perfect ending to the show would have been Buffy giving her life to save the world. While I did not hate season 6 and loved the musical episode in season 6, I just think that had it ended with season 5 the show would have probably never would have been accused of getting stale and holding on too long. In my opinion, season 3 was the best of the series. It had the best big bad(s) in the Mayor and Faith, the storyline behind Angel leaving the show, and really started giving substance to the supporting characters. I also very much liked season 4, which dealt with the core group going their own ways, and then having to come back together at the end. While that season saw the departure of Angel and Cordelia (aside from a couple cross-over episodes with the spin-off series, Angel) and the introduction of Riley and the Initiative, it was really about high school friends growing distant as they go to college. It also saw Anya (played awesomely by Emma Caufield) become a series regular. I know some people really hated the direction season 6 went and it did go much darker than many of the previous seasons, at least in terms of the main characters. The nerd trio was kind of lame, but I liked the direction they went in the last few episodes of that season and I do think the 7th season did a good job of wrapping up the story (especially after Firefly was canceled and Nathan Fillion came on).
Chances are anyone reading this by now is a fan of the show, or at least has seen enough of it to know what it is about. For anyone who is a total newcomer to the show, while I cannot say for certain whether you will like it or not, that is totally subjective and depends on your taste. I can tell you it was a funny, dramatic, well-written, and well-acted show. If you are a fan of Joss Whedon shows and movies in general and somehow have not seen much of the show (and I doubt that is possible) you will love it, especially from seasons 2-5. Of course, it has come out in recent years that the environment behind the scenes was very toxic, largely because of Whedon's behavior, which definitely sours some people's opinion of the show. I still think the show is enjoyable, but I definitely would not watch the bonus material that features him or listen to his commentary tracks anymore. Ultimately, in my opinion, the show was more than just Joss Whedon and I can support and enjoy the show even if Whedon may be a jackass. If your only experience was the movie, which was a lot more campy than the series, don't let that cloud your view of the series. Even if you are not totally into it in season 1, give it a chance. The series really takes off in seasons 2 and 3. If you were in high school at any point in the late 1980s or early to mid-1990s you will get all the references and jokes. Even if you were not, I think you will still find the show entertaining and far better than many of the shows that have been on the air since Buffy has been gone.