These books are weird. The characters are flat and one-dimensional, the plot predictable and drawn out and the writing, whilst technically sound, is nothing special. Yet there's something about them that keeps you reading, keeps you wanting to read.
It's a complete story, so not reading the final volume would be to deprive yourself. I can't say whether you'd enjoy it or not, but I'd suggest you read it and make up your own mind as to whether or not it's any good. At least then you'll be making an informed choice.
If you don't want to buy, then borrow from your library. It's been a top ten title so most public libraries should have it in stock.