As a co-op experience it appears to offer pretty much everything, and the RPG elements give the FPS gameplay a refreshing feel that the genre really needs. While games like Mass Effect and this year's Fallout 3 appear to offer action-based RPG gameplay, they are still RPGs at heart with combat to match - Borderlands is most definitely still an FPS that plays like you'd expect, with the added bonus of RPG elements.
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