I enjoyed this one quite a bit. Many if not most games I've played that try to mix tower defense with a controllable character fall flat because the balance doesn't work. In this one, though, you can't only focus on one aspect. If you just level up your weapons a ton, you'll run up against cooldowns and ammo restrictions and never be able to gun down all the enemies yourself. If you only build towers and try to take a smoke break during the level, you'll be overrun during waves that have skill-dependent enemies.
The inclusion of enemies with a weak point that the player can aim for, but the towers won't, is an inspired choice. It forces you to build and act in concert with your defenses. Tough enemies can be slowed down, things that are only vulnerable in a single spot can be sniped, and swarms can be intelligently targeted with AOE attacks. The sole drawback is that you can only hold 3 guns, and the sniper rifle and cold gun seem, to me, indispensable.
The other complaint I can make is that the 3-D nature of the game isn't fully supported/exploited. So, unlike most TD games, a tower can't just shoot anything in its 'range'. It has to have line-of-sight (in most cases). This is good, as it forces you to think about where to put your blocks, which ones to make shot-transparent, and what obstacles in the level will affect your fields of fire. It's bad, though, because the game will show you a circle on the overhead view that denotes the tower's max range, but nothing else. If you want to learn what it can actually see, the only solutions are to put your character's perspective as close as you can to the tower's (usually impossible, since you can't stand inside it, and the towers are mostly a lot taller than your PC) or observe what it shoots at during the levels (which requires you to look carefully at the overview while the action's going on, and not
participate in the defense). A feature that gave you the POV of the tower would make this so
Of note, this recommendation includes the free add-ons unlocking new towers and weapons, as well as the paid-for DLC granting new maps. This adds much-needed variety, and I would probably be less enthusiastic if I'd played it at release.