I love the Espers and used them when the situation needed it. By end game, I found them more useful than Quickenings cause apparently I have the worse luck with them on this forum...

Summons work like earlier summons in the series (pre-FFVI) in which you can't be expected to use all of them all the time cause they become obsolete fairly quickly. Still, Ultima, Zodiark, Exodus, and Chaos are all incredibly useful and hold their own fairly well even by end game. The Espers are useful as long as you know how to use them. They are "glass cannons" so to speak in that they murder everything around them fairly well but can be killed quickly if you are not careful...

Except for a few I feel its safe to say they are not designed for crowd control as much as they are designed to take advantage of enemy weaknesses (hey just like the pre-VI summons ) Most are worth it if you can fulfill the prerequisites for their special attacks (some of the best in the series) In fact Zodiark not only has one of the strongest attacks in the game but his is the easiest to get off in the game. You can even set it up so Zodiark prepares his ultimate attack as soon as he is summoned.

I like how they are set up except for one major flaw... Most of the summons require their summoner to help them from being swarmed or flat out killed but its difficult to do so if the summoner blows all their damn MP to summon the esper. Had the Esper replaced the summoner and fought along side the two party members (even one if specs couldn't have all three of them on the screen. Just not the summoner.) who actually have MP and crowd control abilities then I feel they would have been far more accessible.

One cool trick is to use them to level up a single character while farming for loot. The Esper gains no exp. so the summoner gets all of them. I also used Ultima alot in the Zodiark tunnels cause everything their is weak to Holy and her Holy attack is faster than the actual Holy spell.

Overall, I feel they are pretty good they just require a different strategy of use than previous games. Personally, I'm just happy XII stuck with X's way of doing summons cause I feel this system works better with the concept of "summoning". Before X, summons were just overpriced, over-bloated black mages...