This is a pretty entertaing solo or co-op Tower-defense/shooter game.
It has a lot of the same fun things going for it that Orcs Must Die! 2 has but it's more serious than OMD!2 is. I think it's pretty good if you're interested in that sort of play. With the three DLC which have been released so far, it has at least 40 hours of puzzle-solving and exploration before the game devolves into grinding for Survival times.
Sanctum 2's solo and co-op gameplay is an evolutionary step away from Sanctum 1. The primary differences are in scale and player utility. That is, the scale is more intimate (close and smaller) while the player characters are specialized for certain utilities (sniper vs. AOE vs. close-range).
Although it's still a Tower-Defense game blended with FPS elements, they philosophically moved away from how the first game worked: 'dozens of weak towers to kill most of the Lumes while the player focuses on a few key enemy-types'
.... towards 'a dozen-ish strong towers which kill about one-half of the Lumes while the player uses special abilities to either help the towers take down all of the Lumes or kill special ones'
Rather than every player being exactly the same, now there are four (five if you have the first DLC) different character classes to choose between who have different specializatons (such as sniper, rocketeer, soldier, and close-quarters).
The developer is very open to changing the game in ways the vocal fan-base seem to want it. When S2 first launched, it received positive reviews by critics, but many fans of the original vocally didn't like it, because it had changed quite a bit from the S1. Since then, the developer has modified the game to mollify the fans (allowing 15 towers per map instead of 10 like at launch, as well as other changes).
The developer is also continuing to add features free of additional charge such as the server browser for co-op players.
(Updated 1 Dec to account for current DLCs)