I'm running 5 druids - the main dual-spec'd as feral (bear) and moonkin, and the other 4 of moonkin. I also run another team with a dk tank and 4 ele shams.

For PvE, the 5 druid team are quite viable - I haven't had any trouble clearing heroics with them. The fights are typically more healing intensive, since you don't have healing stream providing the passive 1K/tick, and no grounding totems to eat incoming spells. However druids bring a few goodies to the fight with Treants every 3 mins, Starfall every 90 secs and great AoE in the form of Hurricane. Tranquility is a great "oh crap" AoE heal, and having 4 available is gold. The only trouble I have is on boss fights where adds spawn during the fight - without an AOE agro generating ability like Death and Decay or Consecrate, you have to be quick on your toes to pick them up before they get to your other toons.

For PvP, druids definitely fall behind Shamans IMO. The the main drawbacks are:
- lack the burst compared to what the shamans can put out
- vulnerable to AoE fear
- have to drop form to heal

Cheers,
S.