I have been intrigued by this novel for years, but I only knew Udolpho by reputation until I finally read the novel recently. Many studies of Gothic fiction cite Radcliffe's novel as a classic Gothic text, one of the early examples that set the standard for the genre as we now think of it. Scholars of the Female Gothic subgenre in particular point to Udolpho as an early example, mostly due to Emily St. Aubert's perfect turn as the helpless female heroine who became a stock character in early Gothic fiction. Then, of course, I read Jane Austen's Northanger Abbey in a college seminar and imagined Udolpho to be a laugh-worthy, melodramatic, fake horror fest. I can't say there aren't any laughable moments (Emily's poems), or that there isn't melodrama (lots of fainting; the parting scene between Emily and Valancourt at the end of Volume I), or even that there isn't some fake horror (all of the "mysteries" are explained by the novel's close); however, Radcliffe's novel defied my expectations in more ways than it reaffirmed them.
The Oxford World's Classics edition with the introduction by Terry Castle is the only edition I've read, but I recommend it particularly because of the introduction, which I found very interesting and insightful after finishing the novel. One point that Castle makes is that despite the novel's Gothic label, Udolpho is more like "a disconcerting textual hybrid." The multi-generic nature of the novel is one of the features that most surprised me; it takes quite a while for Emily to become imprisoned in Udolpho and what precedes her time there is almost anti-Gothic. Emily has perfect parents and the perfect upbringing, though she begins to suffer relatively early on when her mother dies. After this point, she and her father embark on a long trip across France, described at length by Radcliffe in what Castle terms "a bizarre quasi-travelogue." Here we get super-detailed descriptions of natural scenery and of the innate goodness of the St. Aubert clan. Yes, some of the nature described could be filed under "sublime," and such descriptions are standard in many Gothic texts. They are also standard in many Romantic texts, and while the overlap between those two genres/movements is significant, for some reason the Gothic has been viewed as the dark, popular (ew!) sibling of the (maybe) sunnier (self-satisfied?), high-art-producing Romanticism. While the St. Auberts' innocence and goodness make them prime targets for our evil Italian villains (Montoni, primarily), they do spend a lot of their time happily exploring nature, and even after several tragedies befall her and dampen her spirits (and make her faint a lot), Emily is relatively cheerful at times. In other words, the mood is not always Gothic in the novel; indeed, it's probably Gothic less often than it is something else. And then besides the travel narrative, there are also those poems that Emily composes on a whim, about sea nymphs and weary travelers. Radcliffe also incorporates excerpts from poetry into her prose, along with lines from Shakespeare plays, and she begins each chapter with epigraphs from other works. I think that in many ways, the mixing of genres in the novel ultimately makes it a more interesting and more complex text.
Udolpho is a very long novel (almost 700 pages), but, as an insanely popular best-seller in the late 18th century, Radcliffe's work was apparently quite a page-turner. Even Austen's Henry Tilney admits that after hijacking his younger sister's copy of the novel, he "could not lay down again; I remember finishing it in two days--my hair standing on end the whole time." For modern readers, there's not going to be much in Udolpho that is particularly scary, but Radcliffe does create suspense by introducing mysterious plot elements and not resolving those elements for, literally, hundreds of pages. But because all of those elements are, indeed, resolved, and any potentially supernatural phenomena are explained away, the novel isn't really about scaring the reader at all. Instead, we are invited to witness, as many other reviewers have noted, the coming-of-age of the heroine, as she struggles to overcome her passion and superstition to live a life governed by reason and logic. At the same time, however, I agree with Castle that Radcliffe aims "to reawaken in her readers a sense of the numinous - of invisible forces at work in the world." These forces are not exactly supernatural, though; instead, "Radcliffe represents the human mind itself as a kind of supernatural entity." In this sense, Udolpho is truly a Gothic classic as a result of its interest not in mysterious external forces, but in the way in which the human mind registers such forces, and how it attempts to understand and work through them. The Gothic's preoccupation with human psychology is more often-commented on in response to American Gothic works like Poe's short stories or Female Gothic classics like Gilman's "The Yellow Wallpaper," but I see this as a primary interest of Radcliffe's in Udolpho, as well.
I have given the novel five stars, which reflects my personal enjoyment of the work and my interest in the themes and issues it raises for a reader. It will probably be most well-loved by those interested in Gothic fiction, literature by women, and those who are enamored by lengthy, patient, meticulously-detailed narratives. As a fan of all of those things, I recommend the novel and its introduction very highly.