TLDR: Worth it at almost any price, if you like any sort of TD games.
DG:TA is a the perfect execution of a standard tower defense game. As usual, the first few missions unlock new towers as you go, but you have all tower options available for the vast majority of the game (rather than only the last couple levels like many TD games). There's no experience points or carry over from level to level. All towers can be upgraded twice, increasing their range/speed/power. Tower variety is really good.
Story (8/10) - For a tower defense game, this is probably the best storyline you'll ever find. It's touching, believable, and satisfying. The voice acting is superb, which really helps with the immersion. While I've not played more than a few hours of Portal, even I can appreciate the ties to the game in a hidden bonus console, as well as an entire DLC devoted to it. The game is worth playing through on the easiest difficulty simply for the storyline.
Gameplay (10/10) - Levels are very well balanced. Zoom features are great. There's a "rewind back to a checkpoint" button that autosaves every few waves (and you can rewind back through multiple checkpoints). Different levels force different tactics, so you'll use almost every tower at least a bit. Artillery can miss. Rewinding can alter the rest of the mission simply because it resets where all the bullets are, and some may/may not miss. Fast forward feature works flawlessly. There's both fairly set path levels, as well as completely wide open levels that you can maze up to 'til your heart's content.
Replayability (11+/10) - Every level has a whole mess of "challenge" options, such as "out of bullets" (no towers that use bullets), "reverse" (level is reversed), "hard", "no upgrades", "poison" (the objects you're defending kill the enemies if they're dropped), and on and on and on. There's a leaderboard, and multiple medals for each different option on each different map. This is literally hundreds of hours of content. The DLCs are equally as impressive, with a stronger showing of "open concept" levels, they are much harder and present countless different ways to win, and it's rewarding to replay them to find those ways.
Sound (7/10) - The effects are good, cannons and explosions are satisfying, ambient noise/music is fitting although you'll probably turn it off if after the first 30 hours (which is pretty darn good anyway). I put the voice acting under the story, but there's fairly consistent comments from your sidekick during the missions that somehow never got annoying even after the millionth time I heard them.
Graphics/UI (7/10) - Solidly above average for a TD game of its age. You can zoom in quite close, or zoom out for a view range that's more than far enough. Things look like they're supposed to, enemies are all easily distinguishable. Faster enemies will sometime "outrun" where something hits them, so the bullet hitting animation can be slightly behind an enemy (fairly minor problem). The levels show a really good variety without ruining the believability. Menus are very well laid out, as is the ability to see at a glance how you did on each mission (great if you're trying to get all the medals). Nothing here will blow you away, but nothing will annoy you, which is awesome for a TD game.
Achievements/Steam Cards (10/10) - They are there, and work flawlessly!
Overall (9/10) - For what it is, it might very well be 10/10. I, personally, enjoy continuity between levels, which is the only thing I would have changed to make it perfect in my eyes. Questions like "Is clicking on specific towers easy, or is mis-clicking common?" are questions that just won't come up in this game. There is nothing that will detract from the fun of this game, nothing clunky, nothing annoying. You will like it, or not, based on the game concept itself, because the execution is simply done right.