Very good. It's published by a major university press (Johns Hopkins), which means stability, and which also publishes some of the premier journals in related areas (Journal of Early Christian Studies, TAPA, American Journal of Philology). It's got a lot of big names contributing to it.
Is a more objective perspective possible? Well, journal rankings in the humanities are usually regarded as untrustworthy at best, intentionally fraudulent at worst (like in the 2010 Australian ERA review). The only measure I can think of that isn't totally arbitrary and political is the Google Scholar rankings, which are based on counting citations. Here are some numbers that I pulled from there: h5 refers to the largest number n such that it has at least n articles with at least n citations each, and only looking at publications in the last five years --
| Journal | h5 index | h5 median |
|---|---|---|
| Journal of Early Christian Studies | 6 | 8 |
| Greek, Roman, and Byzantine Studies | 6 | 6 |
| Antiquité tardive | 5 | 6 |
| Journal of Late Antiquity | 5 | 5 |
| Early Christianity | 4 | 6 |
| Journal of Early Christian History | 4 | 5 |
| Byzantine and Modern Greek Studies | 4 | 4 |
| Hortus Artius Medievalium | 4 | 4 |
| Byzantinische Zeitschrift | 3 | 4 |
| Zeitschrift für Antikes Christentum | 3 | 4 |
| Byzantion Nea Hellás | 2 | 3 |
Just bear in mind that rankings of this kind are never reliable, especially there are several relevant and important journals that Google Scholar doesn't index. But it's an indication.
I had thought Byzantinische Zeitschrift was the premier journal for Byzantine studies, but JLA comfortably outperforms it in these terms. But JLA is in a fairly unique niche: it covers earlier periods than BZ does, and it's much more specific than GRBS (which covers every period of antiquity as well as late antiquity and the Byzantine era). Antiquité tardive is about the only comparable journal in the Google Scholar index, and they're on a par with each other in terms of citations.
Good, I suppose? It is sort of a difficult question to answer because the name of the historian will often matter more than the journal they publish in, but it has carried articles by people such as Peter Brown, so it is hard to get more legit than that.
As far as I know there isn't really anything else in the English language that compares to it regarding its topic. Its "newness" is mostly the result of the "newness" of the field, at least relative to other branches of classical scholarship.