Its kind of hard to review Cataclysm as a product because the vast majority of it content is actually free in the form of the Shattering patch which everyone got regardless of whether they own just the original game and/or any of the expansions.
This free patch changed the majority of Azeroth for the better. Entire zones have been changed in appearance, some zones have been broken up into two, low level questing has been improved from 1-60 and so on! Basically bringing older content into a more modern era. Improvements to guilds and so forth! These are all awesome and amazing changes which make older content great and replayablity far better!
The problem arises because this means that by purchasing Cataclysm you are only buying access to the lvl80-85 content (i.e new higher level quests, dungeons, raids, gear and zones), the ability to fly in Azeroth, the new Archeology skill and some re-vamped older dungeons having a new hard setting added. This could be argued as being a small amount of content to actually buy!!
So the question is whether or not that paid content is worth it?
If you are a long running wow player, who has a lvl80 character or two and you have done everything else, then YES it is. You will no doubt love the new areas, quests, dungeons and raids etc etc. You will love the ability to fly in Azeroth (and it is cool to do it) and perhaps pass some time digging things up in Archeology.
There is no denying that the lvl80-85 content is fun to play through and worth doing so. But if you are a long runnin WoW player then you already know this, and you know what to expect as you will have bought BC and LK already!
And to those who have said the leveling is too fast in cataclysm from lvl80-85, I would say its fine. If you play casually, maybe an hour a night (while having a healthy social life away from the PC) then you are looking at a good week or two to reach lvl85 and THEN you have tones of other content to do anyway.
But you have to realise its ONLY 5 levels! That would never take long regardless of what Blizzard did.
Personally, I play casually and after about a week of playing the game, I have gone through two zones now, exploring every inch and doing EVERY quest as I find (for the achievements) I am still on lvl83 and only just entering Uldam. That is about 1-2 hours play a night and maybe 3 times a week. I'm not someone who plays WoW EVERYday for endless hours.
I've found the leveling up to be spot on!
Back to my other point.... if you are new to WoW then the bottom line is buy the Battlechest with the original game and BC in FIRST! give it a try, see if you like it. If you do, then buy LK and if you like that, then get Cataclysm.
A warning to new players though... you are looking at 85 levels now to play through, dozens of dungeons and a lot of raids! Having that much content before you WILL be dawnting and it will feel overwhelming and you may become discouraged. But as most people will say... the best content is at the end of the game! If you stick at it, and eventually play Cataclysm, it will be worth it!
If you are new to the game you will find lvl 1-60 a much more fun experienced then even we had when we first startted due to massive improvements. But those aare free anyway.
If you are an existing player of WoW you know what to expect, and you no doubt already made the choice to buy it anyway.