The single player is sweet but for me the game really shines in co-op and verses modes.

I'm not going to copy and paste my full review from my blog because the formatting keeps messing up. You can see it here

Overall Fun-Level: 10 (Highly Addictive)

Graphics: 9 (The best example of XNA graphics available to date. Level design and graphics are to tier.)

Difficulty: 9.5 (well-balanced, nice progression of difficulty. Last levels are a SERIOUS challenge on the difficult game setting)

Re-playability: 9.5 (Commanders and support abilities allow for EXTENSIVE re-playability)

Length: 8.0 (8+ hours on a straight play-through, 16+ if you happen to replay certain levels multiple times, LOL)

Sound Design/Music: 10 (Only one word: Awesome!)

Accessibility: 9 (Approachable for casual and hardcore players. Easy to follow tutorials and camera controls. Caters to all varieties of play styles.)

Mastery: 8 (Difficult to master the high level multiplayer gameplay, except through playing the game extensively.)

Multiplayer: 10 (Very few if any games on XBLIG offer multiplayer of this caliber. Co-Op, Duo-Campaign, Wars, Versus)

Comments: Sol Survivor on XBox LIVE Indie Games devastates any tower or turret defense game found on XBox LIVE Arcade, period. This is the type and quality of game that will legitimize the XBLIG platform. I wasn't expecting anything of this quality or depth on the indie games channel. The extensive single player package, large variety of multiplayer and slit-screen modes justifies the 800MS point. Sol Survivor is highly addictive. Go out with a friend and get it (the multiplayer is fantastic)!

Sol Survivor is a very atmospheric tower defense game that brings some new things to the genre, and I enjoy the game an awful lot.

The main thing about Sol Survivor that lingers after playing it is the mood created. It's one of desolate, haunting beauty the likes of which you don't come by in a game very often, much less an indie game.
The soundtrack, story, graphics, colors chosen and art all contribute to this excellent vehicle that really takes you into the game. The camera control is absolutely unbeatable, allowing you to rotate and zoom every way you'd ever want to (and highlighting the game's excellent graphics engine). I often find myself failing to attend to my duties in the game because I'm horsing around with the camera to check out the action up close and admire the graphics.

Interestingly, you have not only a wheel of types of towers to select from, but a wheel of different player-controlled orbital attacks that are a major factor in taking down enemies. In addition, you select one of three characters to be your general, each with a different lineup of towers and orbital attacks. It keeps the adrenaline level of the game high, and keeps you on your toes.

Some things that didn't hit right with me, though. While I didn't have any trouble with the method of tower placement after some practice, the element of maze building that I enjoy so much in tower defense games is absent (you're not able to modify enemies' paths with tower placement).
Also, some tower types seem redundant or simply not useful - it's difficult to tell why some kinds of towers are more or less beneficial than others, even after a fair bit of experimenting. Some abilities seem a bit tacked on, too - abilities that cause towers to briefly fire faster or focus on one enemy are less useful and less fun than say, dragging the orbital laser across enemy groups manually or firebombing their faces into oblivion.

But that being said, I am enjoying putting a lot of time into Sol Survivor, protecting the colony workers' souls from being stolen. The level of polish, soundtrack, graphics and overall presentation are excellent and I'm glad I picked it up - worth the 800pts.