Gohan's first fight against Super Buu is a fan-favorite moment in Dragon Ball Z. It hailed the arrival of Gohan's ultimate form, dubbed Mystic Gohan by the fans. Gohan's arrival, his vow to kill Super Buu, and the proceeding beatdown are all epic. But then that's it.
For how long Gohan's ultimate form was built up, he doesn't really get to show it off all that much. He gets one fight, then he's overpowered by a Gotenks/Piccolo-absorbed Super Buu. It's once again up to Goku to save the day. Deciding whether Mystic Gohan was worth the hype will require a closer look at what happened and what caused circumstances to unfold the way they did.
Gohan's ascension began with the introduction of Old Kai. After breaking the Z Sword, it seemed like Gohan's chances for fighting Majin Buu had gone up in smoke. However, it turned out that destroying the legendary blade undid its seal on the Elder Kai. With him, Gohan's journey to gaining strength was only just getting started.
Old Kai promised to unlock Gohan's latent powers, but it came at a steep cost-- his valuable time (and a kiss from Bulma). First, Old Kai had to spend five hours dancing around Gohan as he stood perfectly in place. Then, the two of them had to sit in place for an estimated 20 hours. Gohan's great potential meant that this ritual took even longer than usual. The full process took 13 episodes of anime time.
Once the ceremony was completed, Gohan's power became so great that just his Base Form was enough to surpass even a Super Saiyan 3 in strength. Before heading out to fight Super Buu, he bid his father a heartfelt farewell and got a change of clothes in his honor. Finally, Mystic Gohan was ready to show what he was made of. Gohan's entry onto the scene was also awesome. He did an epic superhero landing, casually walked up to the universe's greatest threat, and told him that he wouldn't just fight him, he would kill him. Everything about this buildup radiates so much swagger that many viewers remember to this day.
If the question is whether Gohan's patience was worthwhile, the answer is yes. He made quick work of the Majin and delivered unto him a glorious beatdown. Had the fight gone on long enough, he might have followed through on his promise and, in the process, become the universe's mightiest warrior. Unfortunately, the fight didn't go on. Super Buu self-destructed and beat a hasty retreat. Everything went downhill for Mystic Gohan from this point on.
When Super Buu resurfaced, he took Super Saiyan 3 Gotenks and Piccolo into himself to get an edge over Gohan. This new form ended up being too much for Mystic Gohan to handle and he found himself at the Majin's mercy. Suddenly, Gohan's "ultimate form" didn't seem all that "ultimate."
It was left up to a recently-revived Goku to save Gohan and defeat the Majin. He planned to do this by using the Potara Earrings to fuse with his son into a more powerful warrior. However, Super Buu got to the boy first and absorbed him into himself. This marked the last time Gohan gets to fight in DBZ.
Many fans believe that this switch back to Goku as the main character was the result of Akira Toriyama writing himself into a corner. Toriyama is well-known for having improvised much of Dragon Ball's narrative and it's possible that this is where it caught up to him. With no idea how Gohan could defeat the enhanced Majin, Toriyama might have felt his only option was to reintroduce Goku and a new power-up in the Potara. Goku even commented in-story on how regretful it was that he couldn't leave the Earth's fate up to the next generation as he intended. As for why Gohan didn't just go Super Saiyan, that's another story entirely.
Mystic Gohan in DBZ may have been short-lived, but it's not all bad. He gets more chances to shine in Dragon Ball Super and even has a major part in the upcoming movie. In the grand scheme of Dragon Ball, Gohan is still an awesome, beloved, and respected character.