Sentinel

Pros and Cons
Pros
 * Massive amounts of Health, making Sentinels fine walls for your troops.
 * Truly expendable
 * Slow

Cons
 * Training Time which increases as you "Level Up" your Sentinels.
 * Slow
 * Too much warp space to be uniquely useful at level 6, as a Decimator or Incineraptor can do anything far better and can be deployed in the same numbers, in addition to being nearly as expendable thanks to troop MBs and the like