I actually knew the poem about Lochinvar long before I had any idea what "Marmion" was, as it was in a book of poetry I had as a child. It is the tale of the bold and dashing Lochinvar, who rescues his love, Ellen, when she is about to be married off to a cowardly wimp. This is a great poem that stands on its own, which is why it often appears in poetry books outside of its original setting in "Marmion".
All in all, though, I really enjoy "Marmion: A Tale of Flodden Field". I've always had a fascination with historical fiction and Walter Scott, in poetry or prose, is a master of it. With great characters, a compelling story, and memorable lines of poetry, "Marmion" is definitely worth reading.