If ESO can give off the vibe of its single player games (big open world, the first person gameplay is solid, random quests/path, go anywhere you want and find something interesting, etc), then I'll buy it and play it. Almost all MMO's give the first month free, so really if it's a $60 game and the first month is free, you have 30 full days of play with just the initial game cost. I've bought games for $60 that I beat in 8 hours and was okay with the purchase. A month of play for $60 is better than 99% of games out there, so I'd be happy with my purchase.
If after a month the game is still a blast to play, then I won't have a problem paying $15 to continue.