Math is Fun!
Since
EACH happiness building will provide a bonus empire wide, we can assume that Rome's ability will be powerful. Besides, can you say library spam?
We already know that libraries give +1

for every 2 citizens. However, recent screenshots have shown that forest tiles produce 2

. (The current assumption is that this is the default, though it may be building enabled, as jungle gets 2

with the University). So, with buildings having maintenance costs, you have to judge whether it is worth 1

per turn in that city for the return on investment.
2 citizens? 1

for 1

per turn. Meh.
4 citizens? 2

for 1

per turn. Better.
And so on.
Obviously, it will probably be worth it to build a Library in the capital (Rome) and the 25% bonus will help all cities if/when you decide to build a Library, but it doesn't seem to be the "A++ building spam" ability people think it will be.
Take Courthouses, another example people have been using. Courthouses now reduce unhappiness in occupied cities. If you don't have any occupied cities, or don't plan to occupy any anytime soon, then they probably aren't worth the X

per turn to keep them running. Happiness is also an empire wide modifier now, rather than unique to each city. (This is because excess goes toward golden ages and other reasons).
So, once again, building a Courthouse in the capital first will help when you decide to start building a bunch of them (before a war or something), but you're going to shoot yourself in the foot financially if you just start spamming buildings.