On paper this game looks rather weak - you run around similar environments blowing up large numbers of aliens, consisting of robots, giant ants and spiders. I have to say though this has been one of my favourite games for a while now, I've finished the first three difficultly levels and struggling through the 'hardest' settings.
As you go through the game killing enemies you gain armour and weapon unlocks which apply universally which means if you play through on easy and then start medium you're in a better position than if you went with medium at the start as you'll have more armour and weapons. This is what really keeps me coming back, I know there are more powerful weapons out there which I'm eager to get my hands on plus the fact I don't really like to be beaten. Weapons are broken down into different groups (Assault rifle, shotgun, rocket launcher, grenades etc.) with large amounts of variation between them - each variant has the same model but they can be massively different in use. For example the Air Tortoise fires off a huge, slow rocket leaving a colossal smoke trail as it leaves the launcher which then takes up to ten second to reload. However, the Fork-TW fires something like 20 small rockets simultaneously which then reloads in a couple of seconds.
The lower difficulty levels don't provide much challenge but the higher difficulties are significantly tougher with enemies that move extremely quickly and inflict a large amount of damage. This can be frustrating as it forces you to 'farm' levels - doing them over and over again to build up more armour and hopefully unlock more powerful weapons. The game is pretty much identical in single player and co-op which can make it difficult (and pretty much impossible on the top difficulty level) if you have no-one to team up with.
There's not much variety in the environments you fight in although they are a reasonablly large scale, combined with some huge enemies, large numbers of giant ants and lots of explosions rocking the screen Earth Defence Force can look more impressive than expected for the budget price.
If you're looking for a game you can just drop into it and have a blast, I'd highly recommend this especially for its budget price. However if you're looking for more depth in a game to play for extended periods of time this is not it.