I've been a bit busy with stuff so I never had time to review Heart of Gold when I first watched it. But here it is. Overall, I liked it. It wasn't particularly assuming, and didn't really break the status quo in terms of One Piece. You could even confuse it with Film Z a little. But it kept a frantic pace, well-developed characters old and new, and overall an interesting story. Warning: Spoilers below.
First, the good stuff. The animation is beautiful and makes up for the shoddy Silver Mine arc. I barely even recognized Luffy because he's usually never drawn in that much detail. Most of the film takes place in a fish's stomachs, gastric acid and all, but I was never grossed out by it. In fact the animators used it to make the setting even more pristine and unique.
The first thing I want to say about the story is that it contains two things that should be seen more often in a pirate manga that aren't. It revolves around the Straw Hats and co. searching for treasure, which they haven't really done since Thriller Bark. And there are TWO naval battles in the film, although admittedly both were very one-sided. Otherwise, however, it was a very by-the-numbers plot about the Straw Hats' quest for treasure, and coming into conflict with a guy who uses them like tools, causing Luffy to get mad and beat him. Even though the plot wasn't particularly new, it was still pretty interesting to me, mostly thanks to its frantic pace. Even the scenes when the characters are sitting around help advance the plot, and it never seems like the special is stalling for time before everyone finds the treasure. The third quarter of the movie revolves around some of the characters looking for the treasure inside a temple filled with trials, which was a pretty good usage of the genre, and it put many of the Straw Hats' abilities to use.
Out of all the Straw Hats, Chopper, Usopp, and Brook really stood out. ESPECIALLY Brook. The guy is just sort of there most of the time, lending his swordsmanship occasionally. But he gets a moment all to himself, which actually involves music. Chopper gets to put some of his physical strength on display, and Usopp gets to demonstrate his sniping skills without having any cowardly moments at all. Then there's the main villain, Mad Treasure. Guy is really simple, he just really wants treasure. So while he's not nearly as interesting as guys like Z or Komei, it was kind of refreshing at the end to have him and Luffy engage in simple fisticuffs without the baggage of justifying their positions and all that. Treasure's Devil Fruit ability was also very well used and should have been canon. Finally, there's the newly introduced protagonists, the father-daughter duo Myskina Acier and Myskina Olga. Some people may have found Olga annoying due to her sometimes bratty demeanor, but unlike other kids it actually makes sense with Olga. She's spent 200 years without human contact and it really shows, but at heart she's kind of sweet. And the 200 years away from people also gives her several silly moments, such as announcing all her schemes out loud. Acier is your typical heroic dad, and I found him pretty compelling for the most part, if not entirely original. His dinosaur suit is the best though.
I guess I should dabble on the treasure that they're searching for, the Pure Gold. The Pure Gold is little more than a ripoff of the philosopher's stone, but the backstory behind its creation helped make it more compelling. It also doesn't take the route of "oh it got destroyed no one gets it" route but does actually end up with someone, who deserved it the most, giving it a fitting end.
Let's go to criticism: Mad Treasure's subordinates Naomi Drunk and Psycho P were utter trash. They showed basically no personality, and Naomi didn't even seem actually drunk half the time, or at least her drunkenness was portrayed in the most boring way possible. Their mere concept as a duo is also awful because Naomi is a woman and Psycho P is a swordsman. This means Zoro has to fight Naomi thanks to Sanji's woman problems instead of actually battling a swordsman. But I doubt it would've changed much due to Naomi and Psycho being laughably pathetic like Ain and Binz were. Naomi, who was portrayed as a pretty good shot in earlier parts of the movie, can't hit a stationary Zoro in their battle. Honestly, Usopp would have been a much better match for her. Meanwhile, Psycho P is a camouflage fruit user, and Toei decided to just make Sanji feud with him in the exact same way he did with Absalom. I'm not sure whether the fact that they ended up half-assing it made things better or worse. And their presence made Zoro and Sanji bad by default, as they were stuck battling these weaklings while the other Straw Hats actually used their personalities and powers in interesting ways. Zoro and Sanji have been portrayed as so strong in these specials that there's absolutely nothing to make of them (though, to give Toei some credit, Zoro's like that in the manga too). Franky is AWOL in most of the special and is only relevant for about a minute near the end; I'd really like to see a special featuring more of him. Nami and Robin were fanservice-y as usual but didn't do anything noteworthy other than obsessing over treasure and reading ancient languages.
Overall I give this special a 7.8/10. It's hardly groundbreaking, and I don't know if I'll ever rewatch it, but it held my attention throughout and I enjoyed it; you could say it had a heart of gold. I do recommend that you see it before watching Film Gold, but it's not a requirement, as its only real tie-ins to the film are Gild Tesoro being built up as well as showing how the Straw Hats manage to get to Gran Tesoro.