My opinion on it kind of sucks because I didn't spend nearly as much time on it during my free month as I had planned, but I'll give my opinon anyway.
My brother and I were pretty excited, even though we're not huge MMO fans. I guess we kinda hoped that it would be like the core games only with some friends, but that was stupid of us. Basically it gave us a game we didn't really ask for. Skyrim/Morrowind/Oblivion with a friend is all I want.
It's not a terrible game but from my experience it doesn't do a lot to separate itself from other MMOs. It keeps the Elder Scrolls leveling system where you use things to level it up, which I'm a big fan of, and there's a good variety of things you can do with your character. And as silly as it sounds, it's nice to have an MMO that doesn't base itself on cooldown periods... so this has good ol' fashioned stamina and magic points.
But I think the best part of The Elder Scrolls is the freedom, whether that's sneaking into houses at night, attacking guards (and getting your ass kicked early), or just simply exploring. Since this is more of an MMO than anything, it lacks that, and to be honest, it suffers.
Maybe it's unfair to judge it this way, but since it's titled The Elder Scrolls Online, I don't think it's such a bad thing to want an Elder Scrolls game.
As it stands, it's pretty average but it has a few good things. Over the past month I could rarely muster up the desire to actually play it, so I guess that says it all.