I like bullets, so any gun with multiple barrels is welcome in my Cygnar list. The sentinel also has a nifty bonus by having the shield guard rule, which helps to protect my 'caster/solos from nasty sniping shots.
Sadly, using shield guard is the only thing it has done on the battlefield (and when you couple that with me forgetting he has it). The lancer usually gets more done on the field than this guy. Not a problem though, as I often take them both.
Recently, the sentinel has become more of a back-up plan. With my opponents busy dealing with tempests and the cyclone, the sentinel gets overlooked, often able to make a decent slam attack.