The movies definitely fall within the realm of tongue-in-cheek comedy, with some deeper themes of family and friendship (especially in the third movie) weaved in. They are meant to be a kind of "stupid funny" and you can tell that the writers and the actors took their work seriously, nobody seemed to take themselves too seriously, which made all the movies work, despite all the movies having a different feel and tone from each other.
Each of the discs has an assortment of extras, including theatrical trailers for the first two movies, two different commentary tracks for the second movie, as well as a retrospective on the second movie that was filmed around the time that the third movie was being made. For the original movie, there was also (I kid you not) an air guitar tutorial/featurette, and for the most recent movie, the main bonus feature was a 45-minute portion of the virtual comic-con panel hosted by Kevin Smith, including the writers, producer, director, as well as Keanu, Alex Winters, Samara Weaving, Brigette Lundy-Paine, and William Sadler.
Overall, the movies are all very good if you take them for what they are. They are not meant to be Academy Award-winning movies, just fun, fairly innocent, tongue-in-cheek comedies. I like that the third movie was able to get many of the serving original cast members from the first two movies back together and even found a way to provide a posthumous George Carlin cameo. I know that there are some people who thought making the third film without Carlin would never work, but in reality, he had a pretty small role in the first movie and even less of a role in the second movie (which really amounted to an extended cameo), so while he was a fun character from the original movies, he was not so integral to the story that they could not make a third movie without him. And, I think that the new cast members who were brought in did a good job with their parts, even those who were playing parts that were recast. For those of us who grew up in the 1980s, the first movie especially will cause a bit of a nostalgia trip and the third movie provides a fitting end to the story. It is definitely worth the pickup for those who still get movies on physical discs.