Crisis on Infinite Earths hadn't been the comics industry's first Company-Wide Cash-In Crossover. Secret Wars got there first. (Well, not before some earlier Contest of Champions mini-series, but never mind those.) It was a best-seller, outselling any other comic in the past 25 years... but it only happened because Mattel wanted to make Marvel superhero figures and requested a tie-in publishing event.
The series got bashed. A lot. Critics called it uninspired and juvenile. That perception became a thousand times stronger the following year if you compared it with DC's far more ambitious and mature Crisis on Infinite Earths, which makes Secret Wars look like nonsense for kiddies (which it is). When Secret Wars II was announced, Marvel's Direct Sales Manager was quoted as saying, "Let's be honest. Secret Wars was crap, right? But did it sell?"
I enjoyed it. I think it's good.
I like its simplicity. It's the kind of straightforward mash-up that works so well in similarly underrated stories like The Three Doctors. The plot's a thin excuse to let the writer have fun with the regular cast, basically. A mysterious near-deity called the Beyonder kidnaps a bunch of Marvel heroes and villains. He teleports them to a "Battleworld", a planet created by the Beyonder in a distant galaxy and tells them to fight each other. "I am from beyond! Slay your enemies and all that you desire shall be yours! Nothing you dream of is impossible for me to accomplish!"
The heroes include seven Avengers (including Captain America, Iron Man, Thor and the Hulk), three of the Fantastic Four, eight-ish of the X-Men (including Magneto!) and Spider-Man. The villains include Doctors Doom and Octopus, Galactus, Ultron, the Enchantress and a bunch of others. What ensues is basically a twelve-issue rumble. This works. It's unsophisticated, but it's a laugh. The X-Men aren't always trusted, especially when Magneto's around. (He's still unambiguously a villain at this point in Marvel's history.) The villains also don't always get on, to put it mildly. Galactus is threatening the whole planet, while Doom's arguably the evil protagonist. (The superheroes tend to be reactive, but Doom wants the Beyonder's power and doesn't believe in letting grass grow under his feet.)
The characters are what make it entertaining. Johnny Storm and the Thing have amusing banter. Thor, being a god, has more in common with the Enchantress than any of his fellow heroes. (I love their florid speech pattern.) The Wasp is an entertainingly vain bimbo. You've got entertaining clashes like the X-Men vs. everyone else ("Cyclops may be a jerk, but he's our jerk") and character details like Rogue having only recently changed from villain to hero. There are whacko beats like Magneto chatting up the Wasp. The Lizard is an unpredictable pain in the neck for his fellow villains and falls in love-ish with... yes, the Wasp again. The universe's second most dangerous being is a comedy milquetoast. Things happen that will have a big effect on the characters... but on a personal, emotional level rather than just being another summer event crossover. Colossus falls in love, for instance, torpedoing his romance with Kitty Pryde back home. (This was deliberate from Jim Shooter, who objected to where Claremont was going with the 14-year-old Kitty and the 19-year-old Piotr.)
There are subtleties under the surface. The characters don't realise it, but Battleworld's been granting their wishes. That's why Colossus and The Human Torch fall for a hot girl (unfortunately the same one), while The Thing keeps reverting to human.
Also, Spider-Man gets a new, black costume. I've always thought it looked worse than his old one, but he likes it. Years later, when Marvel wanted to backpedal on this costuming decision, the black symbiote costume became one of Spider-Man's most dangerous foes, Venom! (In hindsight, though, this creates a plot hole. The Human Torch complains that the black suit's web shooters are "even harder to burn off than your old webbing", but it's since been established that Venom symbiotes are weak against fire. Spidey's new webbing should have been easier for Torch to burn off, not harder.)
It's a laugh. Its heroes aren't always the usual versions of themselves (e.g. Iron Man is James Rhodes, the Hulk has Banner's brain) and there's lots of fun in their characterisation. I think everyone gets a cool bit, even if it's just one panel. (Wolverine cuts off the Absorbing Man's arm. Colossus lifts a spaceship. Spider-Man is startlingly effective in fights, even in this overpowered company. Hulk has that mountain.) It's long and not aiming for sophistication, but it's full of energy and I enjoyed it.
"I've been seeing a therapist ever since the Avengers talked me out of destroying the Earth..."