Full disclosure: all I now about Harley Quinn comes from vague memories of the 90s animated series and the TV show Birds of Prey (which might not be the best DC adaptation ever, it seems). Also, I haven't read the first volume, but it didn't prevent me from enjoying the present one. Power Outage is a mix of single-issue stories and a longer arc, hinted at in the two first storie and unfolding in chapters 4, 5 and 6. Knowledges of Future's End might be handy to fully understand Crappily Ever After, but I haven't read that arc and I still enjoyed the story. All in all, Power Outage is easy to read without knowing much of anything else.
Power Outage, and I'm pretty sure the whole series, is a fun and silly read. Harley is definitely an unstable character with a dark sense of what's fun, but her adventures are treated with joy and happiness, even when it's talking about making people explode and covering part the panel with guts. Harley is a mix of cruelty, innocence and plenty of quirks, and it works very well on the pages. Again, it's mostly silly, and if deeper explanations of her behavior are sometimes hinted at, they are quickly followed by more violence and silliness. We're not in a dark character study, but in pure fun.
The three-part story where Harley convinces an amnesiac Power Girl they are a crime-fighting team is the highlight of the volume. I have no idea who Power Girl, but it doesn't really matter here; knowing she is a scantily-clad super-strong female superhero is enough, and seeing Harley trying to get into her sidekick role is very fun. I liked Harley's take on the costume: clearly, when reading classic comics, it looks like ridiculously revealing outfits are a staple of being a female superhero. And why is that? Harley tries to give an answer: to be able to distract the opponent the Kat Stratford way. I still don't like those outfits, but I did appreciate the joke.
Also, space adventures! Giant pug! Marvel parody! What's not to like?