I really like this set of cards. They are basically cheap creatures that aren't exactly creatures until your opponent makes one (in fact, that's exactly what they are (save for one)). All of these would be amazing creatures if they were creatures 100% of the time (a 2/2 for 1, a flying bear, a 3/3 first striker for 3...) but their amazingness is balanced out by the fact that an opponent needs to play a creature spell (or you need to be dying).
Okay, I'm going to take a quick minute here to talk about Opal Avenger since it doesn't have the same trigger as the rest of these cards. It's still pretty good. It's a nice blocker against aggro and it doesn't die to most burn removal. Now that we're done with that, back to the rest of the set.
I can see these working nicely in a control deck since your opponent only has to cast the creature spell, not have the creature enter the battlefield. Are these going to win an Legacy or Vintage tournaments? I doubt it. Are they fun cards for casual? Most definitely.
Pros: Amazing creatures for their cost
Cons: An opponent has to cast a creature or you have to be dying
Note: Props to Opal Acrolith for being able to dodge most creature removal.