Bulwer-Lytton is primarily made fun of for his overwrought style and prose, and there are trappings of those criticisms here that I noticed, but I found it okay - as well as the fact that it fit the setting of the novel presumably much better than his other works.
The book is essentially historical fiction of the rise and fall of Cola di Rienzo, from the time of his young adulthood all the way to the moment of his death. It very strictly paints Rienzi in a positive light to the point where the book is essentially a paean to him - he becomes a representative of everything austere, noble, strong and rational of the Roman republic, contrasted frequently and deeply with the oppression and vice of present-day (mid-14th century) baronies that now make up Italy in the early Renaissance period.
Much of the novel involves Rienzi's rise to power, his political machinations to unseat the barons, mixed with sub-plots involving other characters, primarily those of Adriano, a patrician who shares Rienzi's sympathy with the common people, and his relationship with Rienzi's sister. There's also Walter de Montreal, rogue knight from the Holy Roman Empire and mercenary captain with ambitions of his own that conflict with Rienzi's.
These factors all come together ultimately to form a very romantic and idealised story - Rienzi, a commoner, is driven to revenge with the flippant murder of his brother by a patrician from the Orsini. He also seeks to redeem Rome from its fallen state and elevate the people, which he does in the first act of the book by securing the merchants with him and forming a militia. He manages to drive the patricians out and forms an independent state of Rome, with public political offices and himself as 'tribune' rather than consul. During this, Walter develops his own plans to take Rome and instate a proper aristocratic and a authoritarian regime. These are the two main philosophies that contrast throughout the book, though they're never truly allowed to conflict, as from the second act onward Rienzi's downfall occurs partly through his own egoism, but primarily through unfortunate happenstance, which seems to mostly be in favor of preserving Rienzi's ultimately pure goals, even as the ousted patricians besiege Rome and succeed in turning the people - represented as permanently fickle - against him, who kill him during a final speech to them.
Overall, it was fun, but as dramatic as you would expect from the author, and takes liberties in extremely wide and sweeping statements that come across in the modern day as alternatively saccharine and strange, like his constant assertions of Italians as wily, swarthy and deceitful to the point that they start to look like an entirely separate species by the end of the book. I'd say it probably wasn't worth the elevated page count, and the characters are mostly vehicles for ideas rather than interesting by themselves, especially the women who are purely passive, and with the exception of Walter, who definitely outshines the protagonist.