There will be no surprises here if you've read the graphic novel of the same name and, frankly, if you haven't then don't bother watching the film and pick that up instead. Fans of the novel will likely enjoy Planet Hulk, as it stays remarkably faithful to the source material, but ultimately there's little new here whilst large segments have been removed to fit into theatrical time constraints.
A lot of the cleverer or more interesting elements of world-building have been stripped out, with much more focus placed on events in the colosseum instead. Again, the core elements are all here, including fan favourites like the Wildebots, but the subtleties of the graphic novel have been lost. Similarly, several of the more enlightening subplots have been removed wholesale. Miek never gets to undergo his transformation as the entire slave race, 'native' culture backstory is cut; he remains unhived throughout, which is heightened by the lack of Brood as a character. In turn this lends much less time to fleshing out the various "evil" races that the novel helped readers understand. More confusingly, much of the Spike Wars is left unmentioned along with any real character development between Caiera and the Hulk, leaving their eventual marriage feeling a little ham-fisted and sudden.
Instead, Planet Hulk spends its time focusing on the fighting which is done well enough to be enjoyable. The animation is fluid, the stakes are consistently increased and nothing feels totally forced or off. I found the swapping of the Silver Surfer for Beta-Ray Bill a little odd, as the former has a greater history within the Marvel Universe in general. It also highlighted one of the largest departures, in that this version of the Hulk is far more rage monster than nuanced alter-ego. The graphic novel does a much better job of justifying Hulk's actions and making him a character you both root for and empathise with. There's a lot less of that on display in the film, which casts Hulk as more of an arrogant, impulsive and self-centred teenager. As a result, the plot loses a lot of the emotional connection and impact, though this isn't as great considering the film chooses to end on a high note, rather than the destruction of the novel. Again, I understand why this decision was made, but it detracts from the story quite a lot.