I first tried to read Ulysses aged about 14 (I was an annoying little boy that way) and didn't get very far. The first three chapters are set in and around the mind of Stephen Dedalus, one of the most ridiculously clever and over-educated characters ever conceived, as he takes breakfast with some friends, teaches in a school some miles south of Dublin and walks along a beach. Along the way, his mind ruminates on subjects as diverse as 16th century underworld slang, his dead mother, and something he calls "the ineluctable modality of the visible" which I'm still struggling with. But he's a curiously ambiguous character, this Stephen; he fancies himself as a poet and rebel but when, on the beach, he picks his nose, he has a quick look around to see that nobody's watching before he smears the snot on a rock. (Joyce likes to poke fun at pretension this way - although he doesn't suggest that Stephen's ideas or rebel stance are completely hollow, either.)
The 14-year-old me didn't get that far. I gave up. It wasn't until I was 19 or so that I got as far as chapter four and encountered a Mr. Bloom, pottering around the kitchen making breakfast, that I started to get a grip. Bloom is one of the most likeable characters in fiction. He's a quiet, rather shy, oddly intelligent advertising salesman married to a voluptuous siren of a wife, Molly. Either you're prepared to go the distance with Bloom, or else cast the book aside with a hollow oath, because he's about to spend the entire day walking around Dublin. Nothing will happen except that a man will be buried, a baby will get born, and Bloom will help Stephen when the latter gets into a drunken fracas with some British soldiers. (Ireland was still part of the Union in 1904, and Dublin was a garrison town. Many non-Irish readers concentrate on Joyce's innovation or wit or technical whatever, but Joyce is extremely historically aware, and Ulysses, like all his other books, is riddled with the traces of English domination. These add to the book, rather than diminish it.)
Readers who like those clanky, tinpot contraptions known as "plots" may get a tad frustrated. Leaving aside Joyce's gifts for parody (a _tad_ too indulged, in my opinion), the, if you like, human interest in Ulysses is in the details of the to-ing and fro-ing between the characters. A quite banal conversation turns out to have all sorts of fascinating undercurrents; Bloom, who is Jewish and therefore even more of an outsider than Stephen, is extremely good at detecting the hints and shifts in the tones of the people he meets. He keeps running into two things that cause him particular discomfort: anti-Semitic remarks, and reminders that his wife is about to sleep with another man.
Ulysses is about language, but that makes it sound like it's some godawful lumbering doorstop written by an English professor. (John Barth, come on down!) It doesn't feel abstract at all; it's full of sights (the band of old sweat inside Bloom's hat), smells (restaurants, horse urine, flowers) and especially sounds (cats, printing presses, trams). I can't think of any other book which transports you so completely to a different place and time. (It might've helped that I grew up in Dublin and knew most of the places that Joyce is writing about.) Borges described Joyce's prose style, at least in the earlier half of the book, as "strong and delicate" and that's a good description.
As the day wears on, the book starts to rumble at the foundations and it lurches with increasing unpredictability from style to style. Joyce is making a point about language; that things are altered by the manner in which we describe them. This can get a bit wearisome after a while, but when it works well - as in the chapter where the doings of a young girl on a beach are narrated in the style of a girl's magazine story - it can be very funny and rather touching. The book closes with a mighty tour de force as Molly Bloom sits up and thinks about her life and her curious husband.
Okay, that's the beginner's guide. My personal opinion? It's the best Irish book, a constant wonder, irritation and delight to read, and a stunning effort of imagination and intelligence by the most significant and most lavishly talented Irish writer. 20th and 21st century Irish culture is unthinkable without it. I'm grateful that it's there. What else is to be said?
There are people who would have you believe you have to wage a massive campaign of pre-"Ulysses" study before delving into Joyce's novel. I've heard it's necessary to read biographies of Joyce, read all of his other literature, read about the history of Dublin, read Greek mythology...even study Dublin city maps!!! Don't you believe any of this. "Ulysses" is perfectly approachable having read none of the above. I admit that reading "Portrait of the Artist" first is helpful, and at least having some passing knowledge of "The Odyssey" won't hurt, but being familiar with these other works will only help you appreciate some of Joyce's nuances. Being unfamiliar with them will not prevent you from digesting "Ulysses."
Now, for the book itself. Is "Ulysses" good? That's become an almost irrelevant question to ask. Do you have to like "Ulysses?" No. Do you have to admit that it is the greatest novel ever written? No. Anyone denying that the book was influential in altering the course of literature would just be foolish. However, I don't think "Ulysses" is the be-all and end-all of 20th Century literature, and the new ground that Joyce broke would have been broken anyway had he not done it first. He was certainly an innovator, but other authors (Faulkner comes to mind) use Joyce's modernist approach to fiction and do it better.
For ultimately, Joyce is a lousy storyteller. Notice I did not say he is a lousy writer. One can't deny the absolute mastery of language apparent in "Ulysses." But Joyce is almost completely unable to connect with his reader. Parts of this novel come close to doing just that, but in between there are vast numbers of pages of dull, dull prose that set out to be as incomprehensible as possible. What was Joyce afraid of? Was he scared that what he actually had to say wasn't either particulary interesting or profound, so he had to bury it underneath layer after layer of obscure allusions and writing styles? I didn't understand every part of "Ulysses," and I don't believe all of these so-called Joyce experts do either, despite the massive amount of critical study done about it. However, understanding every single part of the novel and understanding the novel are two different things, and I believe I understood "Ulysses." And what I found is that it's not the beast everyone's made it out to be, but neither is it particulary interesting or profound.
In short, I would recommend that everyone read "Ulysses," if for no other reason than that you can have an opinion on it. I won't be reading it again, so I guess I'll have to just live in ignorance of all the hidden delights Joyce offers his readers. I neither loved it or hated it---there are many books I've enjoyed reading less and many more books I've enjoyed reading much more. Before reading "Ulysses" I was reluctant to state that I didn't like Joyce's writing, feeling that any opinion about Joyce without having read his masterwork would be uneducated. Well, I've read the damn thing now, and I can state with a very educated opinion: "I do not like Joyce's writing."