I've had a great time with my druid. We started the campaign at 9th level, and I wrote up a dwarven druid. Since we didn't yet realize that adventuring druids were limited to their level in Hit Dice of animal companions, I started with three animal companions - a maximum Hit Die Kodiak bear (Rumbleroar - using slightly-modified polar bear stats), a 4 HD wolverine (Snapquick), and a 2 HD raven (Silverthroat). I invested a bit in mounted combat - the Mounted Combat feat and maximum ranks in Ride, as well as a MW Military Saddle. I mostly used buff spells, particularly Greater Magic Fang, Greater Mark of Earth (BoEM I), Bear's Heart (DotF), and Animal Growth to improve the combat efficiency of myself and my mount. As SavageWombat (my DM for this campaign) noted above, not much can stand up to the wrath of a fully-buffed bear.
It's only gotten worse. I've researched a few custom spells to improve my capabilities in combat. I've done in-character research to pick up some spells from non-core resources, mostly notably Greater Mark of Earth and Bear's Heart. Greater Mark of Earth is from the first Book of Eldritch Might, and grants a touched recipient DR of 10/+1 for one hour per level, or until they use an attack that's built into the spell. It's GREAT. If I cast that on myself and my bear, we can wade through a vast horde of low-power opponents without taking much harm. Bear's Heart is from Defenders of the Faith. With that spell, I can give my whole party (1 subject/level) a boost of +4 enhancement bonus to Strength and +1d4 temporary hit points per level. It doesn't last long, and you take subdual damage at the end of the spell. However, it usually lasts long enough for a fight, and during that period, it's quite useful. A few other spells have rounded out my abilities - Speed Swim, Scent, and Remedy Moderate Wounds from Magic of Faerun are all useful.
Berrik Mountainheart is now a 13th level dwarven druid. His animal companions have been reduced to two - Snapquick retired after it became clear that all his association with Berrik was doing was putting him in danger, so now he just has two. Rumbleroar is a 12 HD dire bear, and Snapquick is the same 2 HD raven she's always been - even after a bit of a problem involving the group's alienist and HIS raven familiar. Still doesn't trust those raven chicks that hang around, evne if they are Silverthroat's. No raven that grows to maturity in two weeks is a natural creature. Rumbleroar has died twice in the course of the game, and has been left behind due to size constraints innumerable times. Sad, really, but inevitable. I should have gotten a Portable Hole to put him in, but that would have been degrading.
For the record, here's how I buff Rumbleroar for the day.
Constant
Rumbleroar wears a Collar of Health +2 (putting his Con at 21) and a Masterwork military saddle. He also wears a set of +2 Studded Leather barding, putting his AC at 22 (+5 armor, +7 nat armor, +1 Dex, -1 size).
Daily
Each day as we break camp, Berrik casts Greater Magic Fang on Rumbleroar's claws and bite, making all his natural attacks into +4 weapons. (He also casts this on himself - it lasts for 13 hours, after all, and he has Extend Spell if they're anticipating night work.) He also casts Greater Mark of Earth on Rumbleroar, granting the bear DR 10/+1.
Expecting Trouble
Should we be expecting trouble in the near future, Berrik will cast a fwe more spells on Rumbleroar. Endurance of the Mountains (+4 Con/-2 Dex, similar to the other such spells in Masters of the Wild) increases his Con to 23 (already has a +2 enhancement from the Collar of Health, remmber), but reduces his AC to 21 due to Dex dropping. Should combat begin, I drop a Bear's Heart on the party (+4 Str, +13d4 temp hp), and then Animal Growth seals the whole deal. This all turns my friendly bear into an engine of nature's fury. Here are his stats at this point.
Huge Animal
HD: 24d8+192+13d4 (318 hp + 32 avg = 350 hp)
Init: -1
Speed: 40 ft
AC: 22 (+5 armor, +11 nat armor, -1 Dex, -3 size)
Attacks: 2 claws +35 (+18 BAB, +16 Str, +4 enhancement, -3 size), bite +30 (+18 BAB, +16 Str, +4 enhancement, -3 size, -5 from secondary attack)
Damage: Claw 2d6+20, bite 4d6+12
Face/Reach: 10 ft by 20 ft/10 ft
Special Attacks: Improved Grab
Special Qualities: Scent, DR 10/+1
Saves: Fort +21, Ref +14, Will +9
Abilities: Str 35, Con 23, Dex 9, Int 2, Wis 12, Cha 10
Skills: Listen +7, Spot +7, Swim +19
Spells Active: Animal Growth, Bear's Heart, Endurance of the Mountains, Greater Magic Fang x2, Greater Mark of Earth. This can be done by a 9th level druid, though the bear won't be dire and most of the spells will have less effects.
Conclusion? Druids are very powerful, thanks mostly to the animal companions they can pick up. A high-level druid can unleash amazing devastation if properly prepared.