Who else could give a more authoritative non-official introduction1 than Kida-sensei:
SnowLeopard で新しく加わったフォント Hiragino Sans GB はまさにそういうフォントだ。
これはヒラギノ角ゴシックの中国語版、正確に言うと大陸の簡体字版で、ヒラギノ角ゴシックとデザイン的な互換性がある。ウェイト（太さ）は W3 と W6 の二つ。漢字の字形は完全に中国の規格に沿っていて、日本のメーカーが作ったフォントでは初めて中国政府の認証を取得した。
In rough summary, Hiragino Sans GB is the Simplified Chinese version of the Hiragino typeface. The glyphs use the forms that are Chinese national standard. It's designed to make Chinese characters look good in Japanese texts, and vice versa.
I have always been a fan of Hiragino. Sometimes I even print documents using the fonts in its family. I usually avoid Traditional Chinese characters that are not covered by them2. With zonble's system-wide font fallback hack, I am now able to use Hiragino Sans GB as my default Traditional Chinese font.
What's my take, as a Taiwanese, in using a Japanese/Simplified Chinese font for Traditional Chinese? I have mixed feelings. But I know these things hold true for me:
While the punctuations in Traditional Chinese should always be aligned in the center, not on the baseline, when written vertically (直書), I don't belong to the traditional school that thinks the same should hold true when written horizontally (橫書). So I'm quite comfortable actually with any CJK font as long as it looks good.
For some period of time I had actually used STHeiti as my default fallback font for any Chinese.
This being said, I don't like inconsistencies—and this makes Apple's Hei TC, while a laudable attempt to offer a modernized Traditional Chinese typeface, unusable in professional settings. It also has other major flaws, most objective, some subjective depending on your aesthetic take.
As long as a typeface has consistent radical components3, I don't actually mind if a character is written according to the Japanese, Taiwanese, Chinese, or Korean standard (the first three in the order of my subjective preference).
I want to say that it's a good thing that a Japanese font foundry would want to come up with a typeface that attempts to make mixed kanji writings look good. It's a shame that hardly any Taiwanese or Chinese font I know of tries to do that. Most of the latter two look good in Chinese only (and often only in either Traditional or Simplified, not even both). Many of them, Apple's Hei SC and Hei TC included, have substandard Japanese kanas. The Roman characters in those fonts also look just so-so (this is, however, also something that Hiragino has space to improve). I hardly know any Korean so I can't speak for the hangul part—although I assume the Korean language is much less dependent on hanja (kanji).
No oxymoron here. Kida-sensei is the chief developer of Apple's Kotoeri Japanese input method. ↩
As a native user of the language it's not that hard. Imagine you must not use the word "oversee" and should substitute it with "supervise". Naturally you can't do this too often and too much, but for titles or short texts it works perfectly. That is, again, if you don't mind the punctuations. I admit that this sometimes exudes an air of exoticism—but that's exactly what I intend to do. ↩
And as long as they are modern—say of the Song (Mincho in Japanese) or Hei (Gothic in Japanese) family. I'm not fond of the Kai or the Li family, but that's another story… ↩