Sometimes they work and sometimes they do not. Recently, I ran across examples of both in the same evening. In the course of ONE EVENING, I watched both Clerks II and Road House 2: Last Call.
Actually, let's say I "watched" Clerks II and "subjected myself to" Road House 2.
Perhaps I should define for you what I think makes a good sequel, and explain why these two films do/do not qualify.
1. A good sequel is a rational extension of the original film. In this case, CII is a continuation of the lives of the characters from the first film. RH2 takes place in an entirely different place, with different people, and an almost entirely unrelated plot. There IS a bar, and there ARE some bad guys, but that's about as far as the resemblance goes. With the exception of a few mentions of Dalton and how he's the father of the main character in RH2 (as well as a few admittedly funny "I thought you'd be taller" jokes) this movie had absolutely nothing to do with Road House. In fact, it feels like it was written as a separate film and then adjusted to satisfy some studio suit. According to IMDB, this was originally meant to star Patrick Swayze reprising the role of Dalton, but he backed out (likely because the damn thing was so obviously bad) and it lingered in development hell until 2006, when it was changed to accommodate a new actor.
2. A good sequel will generally include the majority of the original actors/actresses. Unless the first film was an ungodly trauma to film, or unless the stars have have become SO HUGE that doing a sequel is beneath them, they should want to come back and do a sequel, even if a lot of time has gone by. Obviously, you don't want exactly the same cast--new cast members keep things fresh, as Rosario Dawson and Trevor Fehrman do in Clerks II--but the audience wants to see familiar faces. The Ocean's 11 franchise is an example of a group that did well with consistently adding new blood while still managing to bring back the original cast. A sequel without a single member of the original cast spells very serious trouble. That means they either waited too long or it was soooo bad that no one wanted to get near it.
3. A good sequel will acknowledge he time that has passed between the original film and the sequel. In CII, the whole idea is that time has passed and Dante and Randal have reached a point in their lives where things have to change. The gap between the events of the first and second film are explained as the natural passing of people's lives. The crux of the characters' problems have to do with that time passage. RH2 definitely takes place after Road House--theoretically about three decades later, since Dalton did not have a son (he barely had a girlfriend) in the first film and now his son is about thirty and working for the DEA. There are references to Dalton and his death, but the events of the first film are not even mentioned--they just took the name and referenced it once or twice. Not very creative, frankly.
4. A good sequel brings something new to the table without losing sight of what made the original so likeable in the first place. Kevin Smith has done a good job with this in CII. The things that made Clerks so great were the relationship between Dante and Randal, the conversations they had both with each other and with the people who surrounded them, the pop culture references, and the pure grade A raunch. He's done all that in Clerks II, and has also added some new elements. The familiar has been effectively combined with the new in a way that--IMO--is both comforting and intriguing. RH2 feels as though they've used all the same pieces that were combined to create Road House, but somehow they don't fit together quite the same. The older mentor, the girl in distress who is secretly connected to the villain, the crazy villain who likes to smash things, the final showdown (right down to the car that hurtles up to the bad guys minus driver)--they're all there, but somehow they feel forced. It wouldn't have been a particularly bad movie on its own, but it suffered from comparisons to the original.
Basically, what I am trying to say here is simple:
A. Road House 2: Last Call was an abomination that never should have been made, or at least should not have pretended it had anything at all to do with Road House. They could have called it Bar Brawl: Guns, Drugs, and Pelicans or DEA on Airboats or Punching and Explosions in the Bayou and it would have been the same film without all the undue expectations. Plus, as much as I enjoyed Jake Busey's performance as the villain (and God knows I do enjoy Jake Busey and his giant, terrifying Busey teeth), the acting was nothing special, leaving the whole thing just on this side of blaaaaah.
B. Clerks II was everything I wanted it to be. If you liked Clerks, and if you like Kevin Smith's films in general, you will probably enjoy Clerks II. However, if you DON'T like Kevin Smith's movies, don't bother seeing this one. To try and convince an anti-Smith person to like it would be a tremendous waste of time because it is so very him. The subject matter is raunchy, the language is appalling, and it consists of a LOT of time spent watching people stand around a fast food restaurant and talk to one another. Dante is still a whiny baby, but I loved Randal just as much or more than I ever did. There were some great conversations and quoteable moments, and I also really really liked the scene in the prison, because I think it illustrated well the relationship between the two main characters. Rosario did well as the love interest, and the dance scene was a pleasant surprise. It was also wonderful to see Jay and Silent Bob one more time. (I will say that although I find Jason Mewes a sexy beast, the 'Goodbye Horses' bit was a little disturbing. I suppose it's more funny if you realize going into it that this is something he does to Kevin Smith randomly on a regular basis.) I guess I just loved the movie, but I'm kind of a fan girl. Take that as you will.
Maybe someday I will favor you all with my opinions on the second Boondock Saints film and why it sucks and I think Ghostbusters 2 is my favorite. Do any of you have thoughts on sequels?