Yes, you need to get it on HD. Soooo much better.
Planet Earth wasn't actually written or largely produced by Attenborough (unlike most of his "Life" series, the final of which is appearing on the BBC sometime next year I think; sadly, he's retiring

the last one is called "Life in Cold Blood" and will be about reptiles, completing the "life" series), but is actually probably my favourite series he's been involved in (he just narrates it). It's a lot less educational and structured than the stuff he's written and is more of a "celebration" of natural history, hence the even-more-jaw-dropping-than-usual film work and need to watch it on the best quality TV possible.
epidemic, The Blue Planet was actually made about two years BEFORE planet earth; I have both on DVD as I've always been hugely interested in marine biology; actually, I think Planet Earth is superior in terms of the way it's put together and the quality of camera work, but both are superb.
I also really liked "The Private Life of Plants", a really interesting botanical programme which manages to bring it to life impressively with great time-lapse photography and good research, and any of the "Life" series is worth getting if you're interested in the animals in quesion (Life Of Birds = birds obv, Life Of Mammals, Life in the freezer = Arctic/Antartic stuff, Life In the Undergrowth = insects and arthropods; this last one might be the best as it's the most recent, so the camera work is brilliant). He's been doing programmes like this for about 40 years.
Anyways, cliff notes - you need to get "The Blue Planet". And a decent HD TV.
Xas, you need to get The Blue Planet