Gratuitous Space Battles ($23, free level-limited demo) is a game about explosions, lasers, more explosions, fusion beams, collapsing shields, a few more explosions, and tactics. A lot of tactics. It's the tactics that determine if the explosions will be happening on your ships or theirs. Let's get this one totally out of the way up front--it does not matter how fast you can twitch. There is no reflex or speed or reaction time element to this game at all. Once the battle starts, you are a patient observer, no more able to alter how the situation unfolds than Eisenhower could stop time during the Normandy invasion and reposition some troops. You can speed up, slow down, or pause the battle, and throw in the space-towel if you don't want to wait for the bitter end, but you have no control over your ships or their targets once the fighting starts.
Gratuitous Space Battles is all about your skill as ship architect and grand fleet admiral. You design ships, beginning with any of several empty hulls (both new races and new hulls are unlocked through play), and fill them up with weapons, engines, and, if you're feeling unduly compassionate, shields and armor. Ships include tiny fighters, expendable frigates, and massive cruisers. Component choices force you to make constant trade-offs between expense, damage, weapon speed, energy and crew consumption, and many other factors. Fortunately, the interface is quick and easy to use, though the micromanagers in the audience may wish to design massive spreadsheets to optimize everything. I just toss some weapons in and hope for the best.
One you've made some ships, you get ready to fight. There are several scenarios, with more unlocked as you play, but they all boil down to "You have so much money and so many pilots. Build a fleet." Each ship in your fleet accepts generic orders--focus on cruisers, protect this other ship, try to stay back if you're wounded, and so on. In battle, you cannot set targets for your ships or change orders, so setting up the overall tactics can be crucial. I have found that changing a few tactical options--setting more ships to escort duty, for example, or ordering the fighter wings to focus first on the enemy frigates--can dramatically change the outcome of a Gratuituous Space Battles scenario.
The actual battles play out like a movie--you can speed them up or slow them down or pause them or throw in the space-towel, but you can't control your ships. Just watch the very pretty explosions, and hope most of them occur on the other guy's side.
Many scenarios have special limits, such as "Weapon ranges reduced" or "No cruisers." Each scenario has three levels of difficulty. Further, you will quickly find one scenario's winning fleet is another's scrap-metal-to-be, depending on the enemy's fleet makeup; an enemy fleet may be more vulnerable to fusion beams than to missiles, or it may be able to clean out your fighters but not dent your cruisers. You will often need to redesign ships for each scenario, though some designs will become your standards.
There is no direct PVP mode in Gratuitous Space Battles. Rather, you can post, or accept, "Challenges" through an online server, basically downloadable scenarios.
I have a few complaints about the reliability and the game design. First, the game does tend to crash to desktop with distressing frequency--usually when switching from one screen to another, which can cause a loss of a complex deployment as you go to edit a ship design. It's never crashed mid-battle, though. Second, given the multitude of ship designs you're likely to make, the tools for editing and filtering them could be better.
Gratuitous Space Battles is inexpensive and is actively supported by developer Positech Games (an expansion pack was released in December of 2009 and more are promised) and a vibrant community. The demo contains just enough to get you hooked--you cannot unlock new weapons or ships, and there's only a few scenarios to play with--but that was enough for me. If you like the gameplay enough to burn through all the explosive and tactical possibilities in the demo, you will be very happy with the complete game.