While Trevor deals no damage, he performs two useful functions. First, he can take the damage from a spell which would otherwise hit you, which is quite useful against many decks, and especially against potions decks, which can do a lot of damage with one spell.
Secondly, Trevor will get you back a lesson from your graveyard. This is very useful if you sacrifice lessons for creatures or other spells, such as potions. In addition, it can be useful even if you have no cards which sacrifice lessons, because if you have taken damage, there is a large probability that a lesson will be in your discard. Therefore, playing Trevor will let you take that lesson. Thus, he can be used to give yourself a free lesson draw, and enable you to sacrifice more lessons, or play higher cost cards.

At a cost of only 3, Trevor can easily be played even after sacrificing some lessons. Or, if you just cant seem to draw lessons but you have taken some damage, he can give you a needed lesson draw, and then serve to protect you from further damage.

In combination with Halloween feast, you can even get him back and use him to protect you from more spells and get back more lessons. The only problem with Trevor is that he is unique, so you probably only want to play 2 or so of him. If your opponent plays him, you can just kill him and play your own. (Of course, so can your opponent)
I would consider Trevor for any creature based deck.

Rating: 4.5

Rating: 2

Trevor is a mid to late-game card to protect you from your opponent's heavy hitting spells. Playing him early will normally nullify the lesson recovery. I'm not a big fan of the toads and kelpies. Used in the right situation, they can save the game, but, if you're playing creatures, you could have already done that much damage, or more, with a boa or Forest troll.

Rating: 2

For spell decks that have a little room, for a few extra cards, you should think about finding room for 5 COMC lessons, Trevor, and a couple of Pet Toads. Your opponent will not be expecting you to use these lessons for those creatures alone. This may lead them into holding creature killer cards for bigger creatures that you won't have (causing them wasteful turn decisions) and you can stop a few damage spells while they thinking - defense.

Being, Unique, means limiting to one 'in play.' That is why if you are going to play Pet Toads, you might as well make at least one of them Trevor (who can't use an extra lesson). Some believe 2 in a deck can be a waste for most decks, but if you use one for prevention and he gets discarded, you could be playing the other one. This will of course come down to whether or not you will need damage 'spell' prevention.

Rating: 4

I probably wouldn't go to the extreme of puting four of them in your deck, but since they usually drop out fairly quickly against spell decks, the uniqueness isn't that much of a pain.

Rating: 3

In an age where you're not really sure what you're opponent is playing, Trevor would come in handy unless your oppenent is playing only creatures. And avoiding a high damage spell such as Malevolent Mixture, or even the card that wins the Match in play can often mean the difference between the win and the loss. And the lesson to your hand often can make a difference as well if you are playing with a lesson eating deck.

I've seldom used Trevor or Pet Toad, but I can't say they don't have their value either. But I usually end up siding em out for something else. I guess all in all, Trevor is a good card, but just isn't my style. :)

Rating: 3.5