So let me preface this review with the following: I like tower defense games. I LOVE great tower defense games, but I generally like tower defense games. They are great to play while you eat food, download music, or generally aren't in a mood to be hardcore.
With that said, I cannot recomment DG2 to tower defense likers or lovers. The price for the game is entirely too high, and the range of tower types and the interactions between towers are too few. I played Starcraft 1 custom maps with more depth than DG2. And I paid less for Starcraft 1. Crap.
I'll be honest, I expected a pretty high quality TD for what DG2 charges. The story mode is cool and all, but for about 8-10 hours of gameplay (no speedup either), it really doesn't justify the price.
Again, to re-iterate, the towers in this game have no cool interactions. There are no tower A buffs tower B effects, there are no 2 of this tower type adjacent to each other gets a neat bonus, and there is really no reason to buy this game. It has but the most basic of tower types (shoot fast, low damage OR shoot slow, high damage... whee), and the temporal tower is the ONLY effect tower, with an almighty and predictable slow effect.
I will note that each tower has some special abilities you can unlock and add to them. However, from the ~ 2/4 I unlocked on each tower, they tend to be "Deal more damage to shielded enemies", or "Target Priority: Strongest". They could at least have made it "Target Priority: Smartest", so it would at least be usable.
Save the money, go buy yourself WC3: TFT or SC2, and just play custom mapped tower defense there. Save your money. (When this is on steam sale, buy it for no more 5.99)