They're worth owning if you like Star Wars, but they are of course, very different from the old movies. Which is a truth too painful for some...
DO THE PREQUELS STAND ALONE?
Star Wars (or A New Hope) was made as a good stand alone movie. George Lucas didn't know if he'd ever get a chance to make another one, so it had to stand alone just in case. Empire took his awesome story and brought the very talented Kasdan/Brackett/Kershner team on board. It had a huge revelation that would shock and stun audiences and ended on a real downer. Return of the Jedi was a fanfare of awesomeness that would bring forth the manipulative Emperor, resurrect the Death Star and have some of the best action sequences from any of the movies to make Star Wars bow out in style. Most importantly, each of the three films stand alone as great self contained stories in many respects.
Contrasting with the new movies: Phantom Menace - good action, lots of exposition (some of it boring), retarded character created for kids entertainment. Attack of the Clones - great action, a bit more exposition (some of it cringe worthy romance), and few complaints otherwise... Revenge of the Sith - ???.... Revenge of the Sith is what the Prequel Trilogy is all about. All three films are one big story, and the first two films are two hour entertainment spots that lead you to the crescendo in Episode III.
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I personally love the prequels, and think they're worth watching AND owning for Star Wars fans, and I'll elaborate on how I feel about em.
THE PHANTOM MENACE
The Phantom Menace starts things off and sets a trend that the other two films will follow: The story starts with explosions and action. The problem with the Phantom Menace is perhaps that it's not consistant in that regard. At it's heart, TPM and Attack of the Clones are kids adventure films - no doubt about it. If I'd of went to see this movie at the age of 9 instead of my late teens, I'd of shit my pants with glee. TPM takes entertaining the kiddies a step further than the old Star Wars movies - it's sillyness incarnate, especially given Jar Jar Binks' prominence. He's basically an acid induced vision of a pidgin English speaking Telly Tubby from another world. He's damn irritating, but I bet the really young kids love him.
The film loses some consistancy because the Phantom Menace has a strange counter balance to all of the action and Gungan antics... it kind of makes the movie drag in largely expository areas. In a normal film, such stuff would take up a negligable amount of time near the beginning of the movie -- but because this truly is a three part story, it runs throughout the film. This alone makes Phantom Menace a lot less 'snappy' than it's predecessors. We're invited to watch Anakin Skywalker's introduction, see what the Old Republic used to look like (galactic society before The Empire), and we're invited to watch the way the Republic's internal politics work. We meet a young version of our favourite anti-Hero and he's naive yet cocky, weak in many ways yet unnaturally strong in the force. The acting leaves a lot to be desired, but the benefit of seeing Anakin this young is that all 6 movies become about his whole life; they become about Darth Vader. The Republic is completely different from anything we see in the old trilogy. The technology, the architecture, everything. The colors are bright and vivid, there's that much sunshine in parts of the first movie that it's almost cartoon-like.... this changes with Attack of the Clones and again with Revenge of the Sith. The pallete should be an interesting thing to watch changing as you watch all 3 back to back. Then there's the internal politics. For the most part, things seem quite boring and irrelevant at first.. when you watch this movie you're basically gonna be left asking a few questions:
Why does the Queen sound like she was played by a Speak and Spell? What have the Trade Federation got to do with the big picture, if anything? Why are battle droids nowhere to be seen in the Old Trilogy? Why is Darth Maul so cool? Why's he not in it longer? Why does Anakin say "Yipee"? Why Jar Jar? Why Gungans? Why George Why?! ....... most of these questions do look like they'll be answered with Revenge of the Sith. Others, well, maybe they're best forgotten.
ATTACK OF THE CLONES
Attack of the Clones changes things up. We still have lots of action, in fact this one has the best yet. There's even a few remnants of Phantom Menace sillyness. But the focus now is firmly on Skywalker, the Republic and it's new enemy. On Skywalker's side there is a romance story of epic hammy proportions, so again there's still an element of the kind of the sunshine and smiles that pervaded Phantom Menace. But the sunshine is starting to fade. We're starting to see the fear that Yoda sensed in Anakin... in this case it's fear for his mother. Bear in mind that this is something which will repeat itself in Revenge of the Sith, and be used by the Dark Lord to turn Anakin... we're starting to see such fear translate into helplessness, anger, hate. Angst, irrational thought, lashing out. It's all beginning to rear it's head... though Anakin is still far from controlling it and using it to harness the power of the Dark Side. He just doesn't know how yet or have the motive to do so. In contrary to such eventualities, he's actually becoming a great Jedi Knight. He's becoming more powerful and more prominent, and the Jedi are trusting in Anakin (and a prophecy he might bring to light) more and more. It's a mixed tone that should sit right at home between TPM and Revenge of the Sith.
Lots of night-time and lots of rain. The latter half of the movie is almost entirely shot in a kind of twilight hue... fitting given this story explains how the Republic is gonna go down to the Empire. We see Chancellor Palpatine using the position he attains in Phantom Menace and begin to spin people to his design. The parts of the movie that follow the Jedi are little investigative tales that never reveal Palpatine's big secret -- what was elusive in TPM, remains elusive. However, we're gleaning more and more of the Dark Lord's plan. A galactic civil war; The famous Clone Wars. A federation of seperatist systems, financiers and trade institutions on one side are convinced to try and use their power and influence. Backed by massive droid armies, they seceed from the Republic and begin to make unruly demands. Meanwhile, Obi Wan Kenobi discovers that someone has ordered that a huge clone army to be created for the Republic, long before it is authorised by democratic process and The Military Creation Act. BOTH of these sides are controlled by the same insidious hands... pun intended. The seperatists are made ever more powerful and threatening, which in turn forces the Jedi and the Republic to use the clones out of necessity. War breaks out.
Some of the dialogue is dodgy, there's one particularly bad CG character who appears on screen for a few seconds (hint: he works for a "Banking Clan"), and some things are really unnecessary (the droid factory action scene for one), some of the arena battle creatures do nothing to alleviate the cartoon/dumb allegations that are levelled at the prequels... but the complaints are minor in the scope of things, and it makes for a very entertaining watch that both improves in various different ways over TPM, and also sets up the third film very nicely.
REVENGE OF THE SITH
What is/was The Phantom Menace really? What is the purpose of manipulating the galaxy into the clone wars? Part 3. What happens to the Seperatists, their battle droids and superior technology? Part 3. How is the Empire born? Part 3. What of the Rebels we come to know in the old trilogy? How does the Rebellion begin? Part 3. What role does Anakin play? Part 3.
Everything leads to Revenge of the Sith, and from what I've read so far it's gonna tie things up very well.