I have mixed feelings about Sentinel...
As a TD game, you have to micromanage a lot of little things very quickly and keep checking the upcoming waves to know how to deal with each one properly, however, unlike most TD games, the strategy doesn't come from placement or upgrading, but rather energy. In Sentinel, you have a set amount of energy (and adding a tower to the field increases that amount by 1) which you can divide among your towers, to a max of 5 energy at any one tower. You can later reduce energy in one tower to give it to another, to focus your attack on a different lane. This is the main mechanic of the game and takes serious micromanagement skills. During each round, random extra money might fall from the top of the screen for you to collect, and dead enemies might drop special icons that allow you to upgrade or buy different towers, and while the game itself isn't terribly fast-paced, all of this happening at once makes for a real challenge keeping up with it all.
Outside of the TD, there's a stage-select map, which I actually quite like, apart from the gates you have to "hack". Every so often you'll come to a locked node that you can't get past without spending a certain number of "credits". Each stage gives you 1 credit for completion and 1 for getting through it perfectly (and bonus stages give just 1, since you have to do it perfectly or fail). As you get to the later gates, you realize very quickly that you'll have to go back to some levels and get a perfect on them if you want to continue. I find this a little frustrating, as in my (admittedly limited) experience with TD games, I've never had to do a round perfectly to move on to the next.
That's about all the bad I can think of for this game, though, and there's plenty of good to mention. First and foremost, it's just fun! The music suits the game very well, going from calm and relaxed to active and upbeat as you change stages, and it's always very unintrusive, letting you focus on the gameplay. Although it does take skill to use properly, I like the energy mechanic as an idea, being able to focus more on one part of the field at first, then spread out your power later, or turn off a useless tower that round to make a more needed tower a bit stronger.
The bonus stages really make you think about every little detail of the game, from which towers to place where, when to have them on or off, how much energy they need, when to sell them, when maxing out their strength isn't a good idea... the list goes on and on, and each one is very cleverly constructed to make you aware of some aspect you may not have known before, making it a very cool way to teach the player without sitting through a tutorial. One example that really jumps out at me is a bonus stage near the beginning, where you have to collect the early enemy drops to upgrade your towers, and then sell them because the upgraded ones are worth more, and that gives you the money to set enough mines to take out the remaining enemies which can't be destroyed by the towers. If you do it perfectly, you will end up with zero money, so there is no room for error, and you definitely have to know what you're doing to get the job done.
For me, $3 is absolutely a bargain. I'm getting a fresh twist on a cool genre, and I'm learning to think in terms of what makes the most effective strategy, not just which gun does the most damage. So far I've played a little less than 3 hours, and I'm maybe halfway through all of the levels, which means I can expect another 4-5 (after going back to do everything perfectly) for my money, which is considerably more fun per dollar than, say, going to see a two hour movie for $10.
It's not perfect, and it's really frustrating sometimes, but it's enjoyable unique, and that's what I look for in a game. For the price, I have to say yes, I do recommend this game.