[jan_Inkepa]

(To reiterate: I'm not very learned in this domain. This is just an amateur impression).

For English it's pretty solid from what I've seen, and the way it presents etymologies as coherently written readable articles is more accessible than Wiktionary, which is a lot rawer. It's nice that it goes back before written sources to reconstructions of older languages (which OED doesn't do IIRC, but Wiktionary does). On the Proto-Indo-European language front Wiktionary is slightly more luxuriant in this regard because you can search through non-English languages as well and explore etymologies a bit more freely because of this.

It seems to lacks citations as to where the info comes from, which is a bit unfortunate, but I guess it's part of its friendly vibe? But it also obscures how people know this stuff, and makes it harder to fact-check. (I'd be surprised if they didn't have the references stored somewhere that's not getting published - I imagine they're something you'd want to keep track of as you're writing the articles).

[I don't use Etymonline very much because I'm mostly looking at relating etymologies between words of different (Indo-) European languages I'm learning/know right now, rather than plumbing the origins of single English words.]

[rsfinn]

In fact, the basic sources used by etymonline.com are mentioned on the site's home page, which also links to this more extensive (complete?) list of primary and secondary sources: https://www.etymonline.com/columns/post/sources

A few of the individual entries do reference one or more of these sources by name; for example, the entry for "better" says "...Boutkan finds no good IE etymology", where "Boutkan" is one of the entries under "Other Sources".