i thinks it sounds great when the bass plays a different riff than the guitar and have it work. Anti-Flag does it really well. their song 'Stars and Stripes' on Underground Network has great bass riffs and it fits great with the guitar which plays simple power chord riffs.
some people can mirror the guitar and make it good. but VERY rarely. i don't think bass should mirror the guitar. it's an instrument to itself and should give a song a whole new dimension instead of making one dimension slightly heavier or whatever.