Not only is this the best 'tower defense' game on XBLA, it's also one of the better games overall.
'Defense Grid' looks, sounds, and plays fantastic. The visuals are spot on and really give the game a sense of respectability that some other games in this genre miss. And surprisingly, the game even has a decent story to tell while going through the campaign. There is only one voice actor in the game, but he does a fantastic job giving you little bits and pieces of the story as you go along. He can be quite humorous at times, as well.
But in the end, it all comes down to the gameplay. And that is where 'Defense Grid' shines. There are many different towers to use and each has 2 upgrades. There are also many different aliens to mow down, each with it's own strengths and weaknesses. Trying to find the right balance between building your maze of towers and upgrading the ones you already have is a fun part of the game. There is some trial and error here, as you will be restarting a lot of the maps after you know what aliens are coming (and which towers work best), but the game is so addictive that you won't mind.
The game just has an overall polish that is rare to find in any XBLA game. If you are a fan of the tower defense genre in any way, I highly recommend checking this game out. I wasn't even a fan of these games and this game had me playing it for months. It's addictive, looks & sounds great, and is a lot of fun. And in the end, when you are paying $10 for a game, is that what matters most?