I wouldn't even feel remotely qualified to make a statement one way or t'other.. I found Bethesda with Terminator:FS, and then anxiously awaited Daggerfall's release. I have barely scratched the (underworld) of Arena, as I never even tried it until last week (though I certainly -thought- about trying it a few times). I strongly suspect, since I found Daggerfall first, it isn't going to really excite me too much.
I enjoyed Arena, but there really isn't that much to do aside from the main quest. I mean, you can go into random dungeons, etc, but there isnt' nearly as much variety as Daggerfall provides. The other thing is that if you like reading the in-game books of the Elder Scrolls series, Arena has none, and is lacking in the Lore department. I liked Arena for what it was, and wanted to get through it just to say that I did and could. At least now I can read about the events that transpired in Arena (in the books in the later games of the series) and say "I was there, I did that".