Dumas's story was originally serialised in 18 parts so the sheer size of the novel is a bit overwhelming. To tell the truth I found it a bit long winded in parts and if it was being published today it could benefit from some enthusiastic editing. Nevertheless Dumas tells a fascinating tale that still holds up. There are also fascinating bits of history in the book - a glimpse at French society shortly after the time of Napoleon.
One word of advice - If you are reading Monet Christo in English don't be tempted by the free versoin on Gutenberg. It is an inferior 19th century translation. The 1996 translation by Robin Buss (Penguin Classics) is far superior. Buss' s modern language is more accessible to modern readers but his translation is also more complete with restored content and copious footnotes.