From the 30 levels (first 15 as guardian-last 15 as swarm), I must say that Druids certainly have the feeling of being underpowered. That being said, the trade off is that they are extremely versatile in combat <particularly the swarm>.
If you want to be able to tank, attack at range, target multiple foes at once and have excellent daily selection, a swarm druid is your way to go. Unfortunately, you will not excel at the norms of combat through your at-wills/encounter powers at lease compared to more focused classes. Truly a jack of all trades class in my opinion (much like what you'd expect a bard to be).
If you want to be able to tank, attack at range, target multiple foes at once and have excellent daily selection, a swarm druid is your way to go. Unfortunately, you will not excel at the norms of combat through your at-wills/encounter powers at lease compared to more focused classes. Truly a jack of all trades class in my opinion (much like what you'd expect a bard to be).