What bonuses do I get when I flank an enemy? What bonuses does the enemy get when he flanks me? Are they the same?
Flanking negates cover, including "full cover", so it will confer a 20% (partial) or 40% (full) difference in hit chance when there is cover. It confers a higher critical chance - 50% base chance, regardless of whether there is cover or not. It applies both ways (you flanking bad guys, bad guys flanking you).