Doesn't stray far as a sequel, but offers much improvement.
All the same game modes are kept around from the original Hexic, and one more is added. Survival mode is a long trial to get rid of as many pieces as you can, without letting the leftover pieces lock in place and fill up the board; Marathon is the standard game of making special pieces, and Timed is just Marathon with a countdown timer.
In Marathon mode, the goal is to create three or more special Pearl pieces and group them together, before letting any randomly dropped bomb pieces explode.
In Hexic, you would form rings of matching colors to make Stars to make Pearls. In Hexic 2, you form rings to make Stars to make Rubies to make Pearls. There is also the new Emerald piece, created by forming a line of 5 pieces, which helps create more Stars. The effectiveness of both ring and line shapes, as well as the new ways of manipulating the board allowed by these special pieces, gives more options to the player, though the additional step of making Rubies still makes winning in Hexic 2 even harder.
The new mode not featured in the original Hexic is Battle, a multiplayer tug of war in which every color match helps attack your opponent. Battle seems to fill a gap where the original Hexic was lacking: A fast-paced game mode, and multiplayer. Unfortunately, the novelty of Battle mode quickly wears off once you realize that it is mostly based on luck.
Some minor improvements as a sequel include much faster falling of pieces, more detailed graphics (though I personally prefer my imagination over the cyber-world in Hexic 2), and a more catchy soundtrack.
In summary, Hexic 2 is fun little game, though not with much difference to warrant buying it if you've already got your fill from Hexic HD.