For 4 outsider RHD, this beastie gives bonuses to all stats (especially strength and constitution), with the exception of intelligence (which is heavily penalized instead). They've got a decent land speed, can trip people they bite, and get Track for free. They are, however, unable to speak.
Their two special abilities are of varying use. Bay is a decent debuff with a huge range you can immunize your allies against, but it's also sonic, fear, and mind-affecting. Shadow Blend allows mastiffs to get total concealment as a standard action, which presumably lasts until they enter an area of daylight.
+0 LA: a bit strong at lower ECLs but fairly balanced at mid-levels.