A fantastic sequel in every sense of the word
The core gameplay from the original game is brought into this one, and then touch ups have been applied. Actually, it is more like a face-lift: The bone structure is still rigid, but the wrinkles and aging marks are diminshed/improved. The game does not lose its identity at any one part during the gameplay. When you are in space, you're blowing things up. When playing the small-world levels, you're blowing things up. When you are in the cycle races, you are blowing things up. See how it doesn't lose focus?
I believe we can all admit that we play the game primarily to see the weapons...like the Sheepinator or RYNO for example. The array of weapons is simply astonishing. You can even load 5 weapons from the original game if you have a saved game file on your memory card. How neat is that? There are so many weapson in the game that, in fact, the upgrades and weapons for the spaceship minigame are MORE than what some complete games offer! A great feature added was the experience bar for the weapons. After a certain amount of usage, the weapon will be upgraded. These upgrades are not puny. They are solid upgrades that dramatically improve your weapon's ability to fire and carry more ammo. In fact, for some, you need to change your tactics on how to use them.
With regards to Ratchet himself, the addition of an RPG-like experience meter also adds more realism to the game. The more bad guys you kill, the more experience you gain, and after a while, you will gain new health bars to fill. Also, the addition of body armor is a nice touch. Again, not an overkill in terms of changing things from the previous title, but enough to say "this is better than the original".
The game is still linear, like before, but the monotony is eased a bit by introducing small levels in between that act as a relief point for those who are tired of the destruction and mayhem (I don't know if that's possible). The spherical world levels add a very innovative concept unseen in any previous game, so it is a welcomed addition. The spaceship levels are nothing to gawk at, as they were in the original game, just not as obvious. In this game, they tend to be challenging at the start, but once the upgrading phase begins, it becomes a cakewalk. Finally, the cycle racing levels shouldn't be a surprise addition either, seeing as how fun the hoverboard levels were in the original. Once again, no alterations to the fun side of gaming, just tweaks to make the game more enjoyable.
What about difficulty? To say the least, the difficulty of the original game's last level is reached in level 5 of R&C:GC. And once you've beaten the game in standard mode, the advanced mode starts off tough from the get go: Your weapons are ineffective, and the enemies attack with higher damage, so it's no walk in the park. All in all, for an experienced gamer of the original, it shouldn't take more than 40 hours for both difficuly modes. It's difficult to resist playing in advanced mode even if you are repeating the same levels again. The specter for TWO new weapons to unlock as well as the weapon mods are too good to pass up on.
In summary, the game is definitely a gem. It has met all the hype it received, and then some. It proved itself as a true sequel while improving on many aspects of gameplay and visual appearance. A very solid game indeed.