[Like all my reviews, this one is pretty much spoiler-free, except for some vague references. But obviously I can't guarantee that the comments are.]
I'll keep this short, since anyone who's into Harry Potter will have already read this book, or will read it soon.
Deathly Hallows is pretty much what you'd expect from the last Harry Potter book. Almost every character and location from the series shows up in one form or another, even if only for a few paragraphs, as if to see them one last time before saying goodbye. It's not arbitrary, though, and each paragraph has its purpose in setting up the inevitable final confrontation. I wouldn't call it a mind-blowing finale that puts every other book to shame (though it has its twists and moments of brilliance), nor would I say Rowling is a flawless writer, but she does do some things better than almost anyone else.
First, she makes characters you care about. Harry is likable and realistic, and Rowling's unwavering insistence on telling everything from Harry's perspective only enhances that. Other characters are full of personality, even if only given a few lines of dialogue. This ability to create fleshed out characters strengthens the emotional impact when any of them succeed - or fail (and there are plenty of failures here). Second, she creates a cohesive world. There is a complex web of family trees, historical events, and magical rules that loop back on themselves and get described from different perspectives, but somehow it all manages to fit together and avoid contradicting itself. Third, and most importantly, Rowling manages to weave these characters into this world, setting up a cascade of mysteries and questions that slowly fall into place as the plot plays out. It's amazing that she can take all this information and form it into a flowing story both within each book and across the whole series. Harry Potter never has that "I'm making this shit up as I go along" feel to it (sorry, I love you Stephen King, but I'm looking at you here), and I almost believe Rowling when she says that the entire 7-book story popped into her head fully formed during a train ride.
One recommendation if you haven't read Deathly Hallows yet: Read the previous books first, even if you've read them before. Or at least Half Blood Prince. Details from that book especially come into play in this one, and although it's not impossible to follow what's going on, it'll be less of a mental workout if you have it fresh in your mind.
I have no doubt that the Harry Potter series will be right up there with Star Wars, the Chronicles of Narnia and Lord of the Rings as a classic, timeless accomplishment in fantasy. Soon, all the scary fan worship that goes along with it will kick in (e.g., Harry Potter conventions, obsession with mundane details, fan fiction, and dressing up as scantily clad characters of the opposite gender). Still, its popularity is well deserved.