How do you unlock Jacob?

[Andrew R]

And is it worthwhile doing so?

[aywee]

You need to defeat Mother (not to be confused with Mom's Foot).
To visit Mother, you need to unlock alternative floors, and for that you need to beat Hush 3 times.

[Feder]

Yes it's worth it. J&E gives very useful item unlocks:
-The starirway (A shop but for angel items every floor)
-Birthright (An individual ability for every character)
-Genesis (Lose every item and choose one of 3 random one for every lost item)
-Vanishing Twin (every boss spwans twice but you also get 2 items)
-Rock bottom (No more stat down)