This review is for those who've already decided they want to read The Count of Monte Cristo (you won't regret it!), and don't know which version to get.
Short answer: see review title, duh!
The Count of Monte Cristo is my favorite book, and I've read several translations, both abridged and unabridged.
Robin Buss translation is the most modern, and reads most fluidly. A quick example comparing this translation with the one found on Project Gutenberg:
PG - His wife visited for him, and this was the received thing in the world, where the weighty and multifarious occupations of the magistrate were accepted as an excuse for what was really only calculated pride...
BUSS - His wife visited on his behalf; this was accepted in society, where it was attributed to the amount and gravity of the lawyer's business -- when it was, in reality, deliberate arrogance...
Buss's work reads like the book was written in English. The two or so times that the work is nearly untranslatable, Buss makes a footnote about it (eg, an insinuated insult using the formal "vous" instead of the familiar "tu"). Other translations just skip the subtlety. The most common translation out there (uncredited in my version) reads like a swamp. Trust me, get Buss.
ABRIDGED V UNABRIDGED
Abridged versions of this book rarely say "abridged." You can tell by the size: abridged is 500-700 pages, unabridged is 1200-1400 pages. Go for the unabridged.
The abridged version is VERY confusing! Pruning 1200 pages down to 600 leaves a lot of plot on the cutting room floor. Suddenly, arriving at dinner are 4 new characters; it's very tiring to try to keep up with the hole-ridden story of the abridged versions. And you know where the holes are? Publishers "clean up" the book by omitting the affairs, illegitimate children, homosexuality, hashish trips, etc.
As an added bonus in the Penguin Classics edition, there's a wonderful appendix bursting with footnotes to explain all the 19th century references, and a quick guide to the rise and fall of Napoleon (crucial to the politics in the story).
Hope this helps. Get the book and start reading!