Stealth is absolutely deadly in combination with firearms as you get critical shot hits that can one shot most enemies, especially with a sniper rifle. You also get the drop on most enemies and can avoid lots of unnecessary grinding.
I played a stealth character and it was fun, although combat could be frustrating. As forgottenlor said, a critical shot with a sniper rifle from stealth could be swing the fight easily. However, those shots still had a reasonable shot to miss. So 60% of the time when the shot hit, I'd win the fight with no problem. 40% of the time the shot would miss, I'd die, and have to reload.