Anyways, one thing you have to keep in mind. From the looks of things, I find that "goodly" creatures tend to have a lower CR rating than an evil version. So, a couatl is actually far stronger than a succubus, a fiend that shares the same CR rating. I'd put them as slightly weaker than a Glabrezu demon, a CR 9 monster, but stronger than a Hezrou demon, a CR 8 critter. Actually, they might be better than the Yochlol, Lolth's pet demons at CR 10, and the Erinyes, CR 12. I certainly would rate a couatl and unicorn at least equal to the T-Rex, the most powerful Beast (CR 8). The CR ratings are a bit messed up, truthfully.
Anyways, the point is - the Couatl and Unicorn are bad asses, despite their low CR. All this before we start getting our hands on archons in future books.
Also, for the record, you have to be careful with Conjure Celestial, because that's generally cast right alongside Planar Binding, meaning you have a good aligned critter who's likely tolerant of you, around for a full day, and not a Concentration effect.
Meanwhile, clerics also have access to the spell Planar Ally, which lets them call down even greater powers, with no CR issues.
