Wow! Talk about cliff-hanger endings! I loved Opal - it had all the great stuff that made me fall in love with the series to begin with and not only did I not want it to end - I most definitely didn't want it to end when it did! It will definitely make the long wait for the next book seem like an eon, but I guess that's a good way to show that the writing/story were excellent.
I have to admit that I wasn't all that crazy about Onyx and I lost some interest in the series because of it. I have shared an equal love of Armentrout's Covenant Series and I was disappointed in the latest release, Deity, in that series as well, so I thought maybe the writing was starting to take a nose dive. I almost didn't bother with Opal, but I'm so glad I did because it renewed my faith in the series and in the author - I'll definitely be picking up Origin and Apollyon when they come out!