A fun shooter that tantalizes and offers some nice substance, but ultimately lacks soul.
Oh sure, the gun play is fun and the larger enemies are without parallel. Some of them are among gaming's most memorable. But the formulaic approach to these enemies soon wears thin (even when you can't beat a particularly enemy time after time after time--the worms come to mind). In fact, much of the game feels formulaic. You shoot easy enemies and feel compelled (not for any real gameplay reason) to collect their thermal energy. At times, it felt almost like some kind of hardcore, gun-toting Banjo-Kazooie game--only with blobs of thermal energy instead of gems.
To make this worse, your character moves at an agonizingly slow pace.
This downfall speaks to the greater lacking in the game. You go from the tedium of shooting pathetically easy swarms of aliens to fighting incredibly challenging VSs, gigantic Akrid, and stage bosses which most often can easily kill you. There isn't much of a happy medium here. And ultimately this makes the game feel shallow.
It isn't all bad, of course. The graphics are enjoyable, though certainly not awe-inspiring. The sound is adequate, nothing more. There are plenty of interesting weapons (although you can only carry two at a time). The atmosphere of the game is intriguing, although the storyline is hopelessly cliched. The multiplayer offers a whole new avenue of playability which many gamers will enjoy.
If you are looking for a decent shooter--and nothing more--you could certainly do worse. If, however, you are on a tight budget I recommend renting this game or replaying GoW or COD III.