I found that the important game-play benefits only arrive at GM level. The druid's two GM skills are alchemy and meditation. The meditation skill lets him accumulate spell points very quickly, but his ability to use them is limited by being unable to get above master level, and the same spell cast by the cleric or sorcerer is preferable. The key ability of GM alchemy is the ability to brew black potions. At first sight, these look valuable for the big one-off ability boost, but if you go into it a bit further, you find they are not all that valuable after all, because as your skill level rises, the steps at which the relevant ability increases get further apart. And finally, the really valuable protecting and damaging spells are the Light & Dark Magic ones, to which he has no access.
So my conclusion was that once he'd made the black potions, his skills weren't good enough to justify his place in my party.
Like I said, my opinion. The Warlock's familiar might be of more value on the Dark Path, but I didn't get that far, so I can't comment.