This review will contain spoilers!
In many sentences:
The Lightning Thief is simply a fun book to read. Its amalgamation of Greek mythology and modern culture is well constructed and entertaining. In addition to being action packed and exciting, it is also quite funny. The attitude of Percy comes across very effectively via the first person story telling. Though it is clearly aimed at young readers, it is definitely well-written enough that it can entertain adults as well.
I decided to read The Lightning Thief at this time because I had just finished a more serious book (I Am a Cat) and I wanted something that would entertain without requiring much effort from me. This was an excellent choice for what I wanted and I think that anyone looking for something that is just flat out fun to read would enjoy this book as well. Having knowledge of Greek mythology certainly adds to your enjoyment of it, but it is not a perquisite. In fact, for many this could serve as an excellent introduction to the myths themselves.
This is actually my second time through The Lightning Thief, and I have enjoyed it just as much this time as the first. I will also say that the rest of the series is just as strong as this one and if you read the first and enjoyed it, you should not hesitate to read the entire series.
"In a way, it's nice to know there are Greek gods out there, because you have somebody to blame when things go wrong. For instance, when you're walking away from a bus that's just been attacked by monster hags and blown up by lightning, and it's raining on top of everything else, most people might think that's just really bad luck; when you're a half-blood, you understand that some divine force really is trying to mess up your day."Rick Riordan in The Lightning Thief