Umm... well, I'd love to say run right out and buy it, of course. But let me give you a little more info. I'm going to assume you're already a Robotech fan.
If you're looking for quality time with characters from the series, you'll be disappointed. Harmony Gold wanted us to create new characters and not use the ones from the series, because they are planning on relaunching the series in the future, something post-invasion. So there are a few brief cameos from Scott and Lancer, but nothing substantial.
It's a FPS, and it's a good one, but it isn't going to stack up to Halo 2 dollar-for-dollar. We had a much smaller team, budget and time frame to do it in. The multiplayer modes are good, but from what I understand there's not a lot of people on Xbox Live playing them, so you'll have to find some friends if you want to get enjoyment out of the multiplayer modes. Actually, this is really my favorite part of the game, as using the cyclone mode in deathmatches adds a lot of complexity to the matches.
It's a game I'm proud of, though since I wasn't in charge of the design there are some things I would have changed. But overall, I think it does what it set out to do, and that is give the player a sense of helping the robotech forces drive the Invid off the earth.
I hate paying full price for any game. I've only bought 3 games at full price this whole year (Star Wars Galaxies, Fable, World of Warcraft). But $38 for a used copy? I think the game's price point is set at $40 new. And judging by sales of the game (releasing a sci-fi fps less than a month before Halo 2 probably wasn't the best decision on Take 2's part), it probably won't be too long before the price drops down a bit.
Bottom line: in all honesty, I think this is a game you will get your money's worth out of, even at $40. However, if you can wait a couple of months, I'm reasonably sure the price will drop a bit. And it would be a great game to pick up for $20-$30.