I stumbled upon this today and it seems like a pretty great deal to get the four documentaries, especially the Special Edition of Planet Earth, for a total of $62.99.
That averages out to $15.75 a piece. The lowest prices of the individual releases, per CamelCamelCamel, are as follows: Planet Earth: Special Edition ($34.99); Wild China ($17.99); Galapagos ($9.99); Ganges ($13.99). That means if you got them all at the cheapest they've ever been you'd still be paying $76.96.
Anyway, just figured I'd pass this along. Most people probably have some of the individual titles, but for those who put off buying Planet Earth (like me) this seems like a great option.