Eight hours allows for a greater presentation of the many subplots and complexities of the storyline. Depardieu (once you get past his size) is excellent as the Count, a man who, as the reviewer from Amazon says, exchanges one prison for another, a prison of his own hatred. The cast is general is very good and convincing, with Ornella Muti certainly the definitive Mercedes. Set design and production values are excellent.
I have only two criticisms. The first and more minor one is that even this long adaptation doesn't have everything. They couldn't, of course, portray everything -- the book itself is well over 1,000 pages and in truth has padded sections -- but some worthwhile aspects of the story were cut, most notably early sequences with the Abbe Faria (who teaches Dante how to read and write) a very famous part that's usually included in dramatizations of the story. The ending, too, is happier than Dumas's. (I should add that some sequences were very sensibly cut).
A more serious criticism is that Dantes's character is too soft. Depardieu is a fine actor but his portrayal allows Dantes to come off far more sympathetic than I think Dumas intended. The coldness and bloodthirstiness of the Count is not emphasized as much as it might (one sequence at an execution has a very different spin from the way it's presented in the book, for example), nor is the extent of his hatred. This is a real flaw, partly because it makes light of the theme of the story, the effect of revenge on the revenger, and partly because the climax is not as effective if we're already on Dantes side. The flaw here is not with Depardieu, who does his job admirably, but rather with the screenwriter and director.
As such, I can't call this the definitive version -- a little more courage on the screenwriter and director's part would've done wonders. Still, it's probably the best we're going to get. It's hard to imagine anybody else lavishing this kind of care and attention on the story.