It was a nice idea, and the co-op mode can be fun, but it has too many faults to be considered good.
RE: UC is a game made to re-cap all the story elements of Resident Evil Zero, One, and Three with a couple extra scenarios thrown in to satisfy people who already played the original games. While it succeeds in that aspect more or less, it fails on so many other ways.
First off, parts of the story were so drastically changed and butchered that it may leave newbies with more questions then answers. The biggest story flaw was that of the Resident Evil 3 portion. This entire portion was taken directly from Resident Evil: Outbreak without a single change to the backgrounds or anything, which begs the question, why didn't they just take Nemesis out and make Kevin and Alyssa the stars of that scenario? It would of made alot more sense if they had.
Second off, unless your playing co-op with a friend the game becomes incredibly boring very quickly. You will constantly be shooting through wave after wave of zombies and monsters that all look almost exactly the same.
Third is that every single character in the game plays exactly like everyone else. How is that 18 year old Rebecca plays the exact same as all-powerful Wesker? They even have the same health amount! The only difference between characters is the counter attack, but all the counter attacks have the exact same result, an instant kill on one zombie.
And finally, the game forces you to get an A-ranking on some missions in order to unlock certain side stories. I have always hated games that forced you to get a good score to advance the plot...that one might just be me though.
So, in conclusion, unless you REALLY like on-rail shooters, or you are too lazy to go find the original games on eBay, then this game is not worth buying. Youtube the cutscenes and you'll be fine.