As other posters have said, Entangle is balanced in large part by where you can cast it. I currently play in a campaign with a druid (who just hit 11th-level,) so we've had access to Entangle for a long time. The very first time the druid cast the spell, the DM was mighty surprised, and we were mighty glad, but after the initial shock, it hasn't been unbalancing in the least.
Can't count the number of times I've heard this conversation:
Druid Player: "Is there any vegetation around? Weeds, moss...anything?"
DM: In the {dungeon, castle, marketplace, cavern, pit, etc.}? Nope.
Everyone: "D'oh!"
Sure, you get good use out of it sometimes, but not often enough for it to be unbalanced. Also, providing they survive their first encounter with it, PCs should be ready to deal with an Entangle thrown their way. As DevoutlyApathetic points out, there are ways to deal with its effects.