It depends on how you evaluate them.
If you consider the ratio of construction cost/requirements/unit cost/characteristics/construction time, then manticores are clearly not worth it.
They have the highest construction cost and requirements for their level. They are even more expensive than nagas.
The unit cost is comparable to that of a wyvern.
The characteristics are comparable to those of a wyvern.
And paralysis is clearly more useful than poison.
But wyverns can be built on the 1st turn (if you have archers in the castle), and with Tazar, they become practically invincible. Wyvern Rush on small maps is effective.
IMHO
This is just my opinion. :)