The Worldwound Incursion is an extremely good start to an epic campaign. This module of the Adventure Path builds a solid foundation on which the rest of the campaign rests.
The start of the module effectively not only shows what is at stake in the campaign and what will happen if the PCs fail, it also manages to build solid relationships with many of those who will be the PCs' closest allies as the campaign progresses. The NPCs have clear, strong and differing personalities which together with their background stories make for believable and likeable (or at least entertaining) NPCs.
Furthermore the AP manages to shine a light on not only the physical corruption demonic taint brings to mortals and nature itself, but also shows how the corruption of crusaders, mercenaries and in general fallible mortals slowly destroys the very nature of the crusades and crushes all hope of victory.
Add that the story is brilliant, the combats appropriately challenging and the rewards are very good as well, and the module offers plenty of good roleplaying opportunities, whether one prefers the more serious, the over the top and funny (with a touch of the dramatic) or a mixture of both.
The only negative I can add is that for any moderately competent group the mythic rules being introduced in the end pose quite a challenge for the GM in future modules. Mythic is overpowered, there is no way around it, and in my group even the suggested alternative stat increases make for too strong a party if one wants to play the entire AP exactly as written. As the campaign has progressed I've needed to increase the CR considerably to keep combats challenging (or just at a point where they drain PC resources), but luckily the Paizo forums have an amazing reworking of higher ranking enemies/allies/neutrals. Personally I find that those reworked stats and the stronger enemies being allowed to use mythic while the PCs aren't makes for an appropriate challenge, but it would depend a lot on how experienced the players are.
All in all The Worldwound Incursion is a brilliant start to a very, very good campaign, although later modules do need a bit more mechanical tweaks from the GM's side than the average AP. The help found on Paizo's forums helps a lot in this regard though.