After the fourth book, which left me a little bit less satisfied than the other novels, I took some time of reading the series. While not completely forgotten, I didn't think a lot of the series and since were library books, it took another three years for me to finally start reading The Last Olympian. And I'm really glad I did read it.
It took me a day. That says enough. I switched to the English books for this one, and although I nowadays prefer reading English I got the impression that the Dutch translation was quite good this time, as I had no trouble to adjust to the English writing, and it all had the same feelings surrounding the writing. The humour also stayed the same, as far as I remember.
It took me a little time get back into the story. Obviously, after three years, not every detail was completely fresh in my memory (the 500+ books I read in that time probably didn't help either), but this wasn't a problem as it all came back really fast after I started reading. I immediately remembered the mythology that made me really like the series to begin with. In this volume I appreciated the inclusion of the so-called minor Gods, like for example Morpheus (after which morphine is called). I've always had a soft spot for them and never understood why they didn't deserve a bigger place. I can't blame them for not stepping up to save the Olympians.
This is the endgame the last books have been building up to. Is the ending a surprise? Not really. Does this matter? Not really.
I'd recommend the whole series.