These types of events are typically very casual, with the intent (from the manufacturer's point of view) being to introduce new players to the game (and more specifically, their product). I wouldn't describe the competition as tough, but things can certainly come down to the last few throws if no one has an insurmountable lead toaward the end.
That said, the atmosphere could vary greatly depending on the tone the Tournament Director chooses set for their event, and how many experienced players decide to attend. Many TD's go out of their way to make these events fun and inviting to newcomers, and solicit experienced players they know to not only attend, but to donate discs and stuff for prizes to increase number of people who feel like they won something. Other's like to keep it a no frills event where the best score wins and not do much else.
Entirely possible for two Trilogy Challenges (or similar company sponsored events like Discraft Ace Race, Vibram Birdie Bash, etc.) in the same town to have very different vibes to them, based on how the event is run and who decides to show up.