Hollow Mask: The Vizard remain in complete control while wearing their masks (unlike Ichigo's early unstable transformations). According to Aizen, the fusion of the two spiritual entities allows these Shinigami to reach power levels far surpassing that of either a Hollow or a Shinigami.

The Vizard use their Hollow powers to augment their existing Shinigami abilities, granting them massive increases in speed, stamina, and strength.

The only known side effect in donning their Hollow masks is the specific time limit, which varies from user to user. Fortunately, there is a remedy to this problem: Training (but to what extent is currently unknown); Most of the Vizard appear to have a limit of three minutes after a century of training, while Ichigo has expanded his limit from 11 seconds to several minutes in a relatively short period of time, so the factors influencing the limits of the masks are still unclear. Every time their masks are used, the Vizard's voices echo as a consequence of both their normal voice and their inner Hollow's voice speaking simultaneously.

Enhanced Speed
Enhanced Spiritual Power
Enhanced Stamina
Enhanced Strength

Cero: Vizard gain access to certain Hollow abilities, such as the Cero blast; However, only Shinji, Hiyori and Mashiro have been seen using it.

Power Augmentation: With the acquisition of the mask through the subjugation of one's inner Hollow, Vizard gain access to an additional and separate source of power - aside from the Shinigami powers they already possess - Hollow powers. The accumulation of both powers greatly augments their powers beyond what a Shinigami and/or Hollow alone would be capable of.

Mask Protection: Body damage can seemingly be ignored while wearing their masks, as demonstrated by Ichigo's Hollow during his unwilling transformations.

Reiryoku Enhancement: Vizard have dual spiritual power of both Shinigami and Hollow, which enhances their abilities.

From Bleach Wikia