Okay, then terrible, then bad.
I haven't read Inheritance yet, as of this review, but I can certainly talk about my experiences with the first three books.
The first time I read the first book, I thought it was fairly well made, with a good story and good characters. Recently, however, I took another glance into the book and realized just how bad it really was. The Shades were bland villains, not threatening-looking. The Ra'zac (I think that's how it's spelled) were pretty good, however. The magic system was so-so. Dragons and their riders? Done before, but it works. But the plot was the worst part. An evil king that the main character overthrows? Not worth reading. Sure, they had the whole Forsworn background, but it, in my opinion, failed to add any sort of color to Galbatorix' character. Eragon himself I found to be a little too childish for the story. I didn't like his constant history lessons from Brom.
Eldest was definitely the worst of the three, about half the time. The only reason I enjoyed that book at all was because of Roran's parts. Roran was a good character, and I liked following his trials as an ordinary man, contrasting with Eragon's bland "adventure", learning "exciting" magic in the forest with the elves. I hated those elves. They were so overpowered and at one with nature, with senses far surpassing those of men, that I almost skipped entirely over Eragon's bits. In the end I read them out of curiosity. But overall, this book was lackluster and boring. Oh, and I almost threw the book out the window when I found out about Eragon's namesake.
Brisingr was better than Eldest by a long shot, but still fell short of Eragon. Eragon's elf powers were annoyingly overpowered. The integration of elves into the story was still just as annoying as in Eldest. And I hated Nasuada so badly when Roran got whipped for disobedience. And when an injured Roran wrestled with a Kull and WON, I was just about to give up. Roran had lost his normality by becoming a fricking SUPERMAN. Eragon's progression into a vegetarian was frustrating at best. He became such a nature freak that I lost interest in the series after I finished.
So, to summarize: The series is annoying and not worth reading, as least up to the third book. Your time would be much better spent reading Tolkien, Rowling or Brian Jacques.