I have a 5 x Druid team at 80, decently geared (hateful / deadly / WG epix)...and they are definitely a lot of fun to play.

Armory Link

For PvP they are defintely viable but I would place them well behind shamans in terms of effectiveness. For PvE they are pretty solid - I have my main dual spec'd feral and two alts dual spec resto.
Pro's:
1. Stealth - great for getting the drop in pvp, or sneaking around mobs in pve.
2. Instant forms - very convenient, especially flight form
3. Great AoE - Hurricane does awesome damage x 5, especially when backed up with Starfall.
4. Good CC - Entagling Roots & Cyclone are pretty good.

Cons:
1. Have to drop form to heal - makes you very vulnerable to melee classes (leather not much better than cloth really).
2. No real mitigation for spell damage - although you get resistances with MotW, there's not much else. You can barkskin for a temporary solution.
3. Vulnerable to melee zerg - outside of Typhoon, no spells / abilities with pushback. Changing to travel form and running is usually too slow.
4. Vulnerable to fear - one trinket and thats it.

My druids are currently my "no. 2" team behind my Shams. They are good to swtich to for BG's & WG when I feel like a change of pace - they are a very mobile team as opposed to the Shams "stand and nuke" style, and are actually my preferred team for dailies (mainly due to instant flight form). Worthwhile levelling IMO.

Cheers,
S.