In short, yes. It was. Before I dive in, I must say I really like the cover (even though I think it's kind of silly when they cut the tops off of women's heads on covers). The story is basically a modern day retelling of Hades and Persephone. I've said before I'm all for fairy tale retellings, and Greek myths are close enough for my tastes to share the same enjoyment.
And now for a slightly more detailed summary: Nikki (protagonist) vanished into the underworld (called the Everneath) for several months, throwing her friends and family into a panic not knowing where she went. Now she's returned - but she only has 6 months before she has to go back for good. The story flips between her past (which allows you as a reader to piece together the events that led to her decision to go to Everneath in the first place) and her last 6 months in the mortal realm. I thought this particular technique added a lot to the book - I found it extremely interesting to read the juxtaposition of the past and present as the story progressed. I think if it had been chronological straight through, the book would have been a lot more bland as a whole. As it is, I was sucked in right away between the ticking time bomb of her impending doom and the mystery of her past. It added a nice balance to the pacing that I felt worked really well.
The interactions between Nikki and Jack (main love interest) were also well done. The awkwardness between them when she first returns is excellently done. The main antagonist, Cole, struck just the right amount of creepy stalker vibe, and though he didn't develop as much as Nikki and Jack, I even felt a little sorry for him at the end. (Only a little though.)
As far as endings go, this book is the perfect example of "standalone with sequel potential". The ending is pretty open, but it also gives enough closure to be satisfying. I do wonder about some of the minor characters (like Jack's mom in particular), but I think going into any more detail would have stretched out the ending too much after the big climax. If there's a sequel, I'll read it; if not, I won't be upset. I think this book works quite well on its own even with the open ending.
Overall, I was extremely pleased with Everneath. I will certainly read future novels Ashton writes. There is absolutely nothing objectionable in this book, but I think ages 12+ will enjoy it most (particularly girls).
It kind of reminded me of Wicked Lovely, which I also enjoyed, but I think I liked this one better. :) So if you liked Wicked Lovely you should probably go buy this one right now.