You can read Ulysses without guidance. You can. Certainly it's likely that if you do, you'll miss out on a fair amount of what's going on in the novel. That's not a bad thing, necessarily, as if you're anything like me, Ulysses is a book you'll come back to again and again, and there will be something new, some whole new thread of theme or motif, some new chain of allusion, to grab you anew. It's a novel, and it's a way of reading.
I've read a few Cliffs Notes, and I've written a fair number of Masterplots entries (and other guides and reference works of that ilk), and in my opinion, if used in conjunction with a careful reading of Joyce's novel, Kopper's little book is a nice first guide. It sketches you a map that will carry you through on your first trip. Maybe, if you have the time and interest, you'll reread the novel with Harry Blamires's Bloomsday Book as a companion, or even Gifford's Ulysses Annotated. And Thornton's Allusions in Ulysses. And-- ...
But the Cliffs Notes are quite good for what they are.
When I first read Ulysses 25 years ago, I did it like this: I read an episode, then I read the Cliffs Notes, then I reread the episode, and then I moved on. That's how you'll get the most out of this tool. If you use it for what it's supposed to be and don't let it read the book for you, you could do a lot worse. When I taught the novel, I was pleased to recommend this guide to my students.
In the interest of full disclosure: I first studied Joyce as an undergraduate with Ed Kopper, but as I haven't spoken with him in at least 20 years, I think you can consider this review minimally biased at worst. Enjoy!