Quick, dirty, and fun. Too bad it leaves you wanting more.
You are only able to hold two weapons (not the issue) but one of them must always be the Gallant. It happens that this weapon does get better as the game progresses and its final form is extremely useful, but that only leaves you with one slot for a weapon of your choice. Most of the time that weapon will either be a pistol or a sub-machine gun and ends up performing the same way as the first two versions of the Gallant. Every so often you will come across a shotgun (with rare ammo, but can kill in one or two hits, something only well aimed sniper shots from the final Gallant can do) or my favorite, the mortar rifle. The Mortar-Rifle uses the same ammo pool as the Gallant, the ever available protoculture and can be charged (and charging the weapon does NOT use more ammo). Even uncharged, the largest of foes will fall in two shots on even the hardest mode. (If it takes you three shots then you should just cry.)
Quickly in the game you will find some Cyclone armor. This powered suit gives you faster movement, and small double-jump/boost power, and shields. You also get a night vision visor, and as the game goes on, thermal visor and even a cloak (which for some reason is counted as a visor, thus disabling NV and Thermal when used. Your version of the Cyclone armor also changes throughout the game (which is how you get the new visors) and some sport an energy blade. This weapon is good when your target is not facing you as many Invid can get you faster then you can hit back and most of the time you will be trying to melee ones that have a shield.
What COULD have been a great feature of the game was the ability of the Cyclone armor to transform into a motorcycle around the player. (In fact, the player first mounts the armor when it is in motorcycle mode and from there it turns into armor.) The problem is that the motorcycle controls are sluggish, your shields are disabled, the only weapon you have, missiles, are slow, and hitting stuff at almost any speed hurts your health and can even force you back into biped mode. The only time you will use this mode is if you are messing around, you have to get someplace quickly, or it is a scripted part of the game that requires the use of it (which is almost the same as getting someplace quickly)
The number of different enemies is actually on par when compared to popular games such as HALO and Half-life. There are about eight different enemies types and a few sport different versions. Compare that to the original HALO's four covenant and three...-ish Flood, plus the flying Sentinels and you got your eight right there.
Level designs are EXTREMELY linear and except for a few, sometimes well hidden, unlockable multiplayer skins, you won't be looking around for anything. Once you play through the game once, (it won't take too long) you will know what to expect, everywhere.
The game difficulty is hard...to find. Really. Even on hard the game is easy. As long as you don't go trying to melee everything in sight and know how to aim a gun, you won't have any problems.
Multiplayer? Never tried it. It is the biggest pain in this game. Support for up to eight players but no split screen. Works on-line, if you can find a server.
To make a long story short, this game is great if you are just looking for a quick and fun FPS (or even TPS as it can change at will) and nothing more. If you can't find it for cheap (at Blockbuster it would have cost me eight dollars to rent a game but they had this in their buy bin for seven) and you expect what you pay for, this go ahead. Robotech: Invasion is fun, but seriously lacking.