If you haven't seen Season One, stop reading this right now and go watch it. You're in for a real treat, as 6FU is one of the most well-written TV dramas ever.
If you watched Season Two when it originally aired and are considering purchasing this set, you should know that the DVD extras are not plentiful. Several of the 13 episodes have optional commentaries you can listen to while you're watching the episode; I find this is a big investment of time for a small payoff, but that's just me. Plus, the last disc contains a short documentary "anatomy of a stiff," which is worth watching once, but not really that compelling. Another factor that might affect your purchase decision is that the episodes themselves are rich and stand up to repeated viewings, unlike most of the tripe on television. Plus, watching the episodes on DVD - on YOUR schedule - makes for a much better experience than waiting a week or more between episodes as they are aired on HBO.
If you've seen Season One but not Season Two yet, the best indicator of whether you'll like Two is how much you liked One. Yes, it's true, fans disagree over which of the first two seasons was better. But the overwhelming majority that liked One a lot also liked Two a lot.
I think both of the first two seasons were EXCELLENT, but a bit different. Almost every episode of the first season was very good or excellent. The second season had the same average quality as the first season, but more variability - that is, a few episodes or storylines were a bit weaker than what we typically saw in Season One, but other episodes and storylines were even better than those in Season One. The last two episodes of Season Two rank among the best television I've ever seen.
I will try to set up the events of Season Two without giving anything away. At the end of Season One, Nate was coming to grips with a newly diagnosed brain condition that could cause seizures, or even a stroke and death. Nate and Brenda had declared their love for each other, despite a fairly complicated and sometimes rocky relationship. David was single and struggling in his love life, and coming to terms with being "out." He and Keith were still friends, and it was clear that they still had feelings for each other. These storylines are developed further in Season Two, with many interesting twists and turns along the way. Of course, there are also storylines involving Ruth and Claire. Ruth's growth in Season Two is nonlinear - she seems to make progress in one episode, but then reverts back to her depressed, repressed, controlling self in the next. Claire has interesting relationships with different people, each of which shapes her in some way, good or bad, and she gets a bit of direction in her life toward the end of Season Two.
One of the things that makes 6FU so great is that it's so real. The characters are 3-dimensional, flawed, and (mostly) likeable. Whenever I watch the show after not having seen it for a month or more, it's like getting together again with old friends. I also respect the show for taking seriously the issues of religion and minority sexual orientation (Amazon.com has previously censored me for using the word that begins with "h"). Half of the main characters are religious, and their views are treated very respectfully. One or two of the main characters are g__ (again trying to avoid the censor) and they are portrayed as real people, with the same joys and sorrows and problems as straight folks.
Bottom line: If you haven't seen 6FU season two yet, you are in for a real treat. Be patient through the first two episodes, though, as their slightly lower quality is not representative of the rest of this outstanding season.