Downloaded the game Undertow from XBLA the day before Thanksgiving and was immediately un-impressed. And this is precisely why reviews and critiques of games shouldn't be based on initial impressions.
The game is an underwater side-scrolling shooter with controls similar to Geometry Wars and the dozens of other games that try to be Geometry Wars. Fortunately, as I continued to play Undertow, I began to like it. Then, after playing a 16-person multiplayer battle, I was hooked. But let me back this boat up and start from the beginning.
The single-player campain has you play through three chapters consisting of 5 levels each. Each chapter is for a different race (Human, Nemo, and Atlantis) and each race has different types of units. All races have the same four classes but feature different designs and weapons abilities. And you can upgrade these units as you continue to score points, however the benefits to doing so aren't quantified anywhere in the game -- you're just left to interpret that a level 3 unit is stronger than a level 2 unit. How much stronger is anyone's guess.
But no matter which chapter or level you're playing, the goal is pretty much always the same: take control of all five control points to reduce your opponent's ticket count to zero. This is done by shooting enemy units, driving them away from a base, and then floating around the base long enough to turn it to your team's color. It's a very simple mechanic that has been done in countless other shooters, particularly the WWII games, but it surprisingly never really gets old. And although my one true complaint about the game is that the enemy A.I. is far more aggressive and intelligent than the A.I. of your companions, it's possible to play the single-player campaign in Co-Op mode over Xbox Live with a friend, so playing through the Ultra difficulty setting for the Achievements is certainly doable.
So, yes the single player mode does get a bit repetitive and so can the multiplayer mode if you're not in the right mood when playing it. But what I really like about the multiplayer mode, particular in a room with at least 12 humans (and only 4 bots) is that momentum can roll back and forth like the tides. You're never truly out of contention in this game and it's not unusual for there to be several lead changes in a single game. The action is frenetic and since everyone is dying so often (and often from friendly fire via Depth Charges) that the atmosphere in each of the game's I've played has been lighthearted and friendly. It's competitive, but polite and everyone seems to have a good time playing the game.
Ideally, there would have been some maps that were a bit more unique or, perhaps, a gameplay mode or two that required different winning conditions, but what the game does, it does well. It's certainly not going to hold my attention as long as some of the other $10 XBLA games, but it's not over-priced and is definitely worth checking out.