Well, where to begin? The scene at Orthanc makes a lot more sense with Saruman back in. Of course, without the Scouring of the Shire it runs differently than the book - but as I thought when I first read about that, I realized that the scouring would seem very anti-climactic on screen. The wrapup with Saruman should have been in the original - it worked very well. Heck, his "brigands and dogs" speech to Theoden makes Theoden's later admission to Eowyn - that it was not him that saved the day at Helm's Deep also makes more sense. There's even a small shot of Pippin being distracted by the Palantir as he and Merry dance on the table - his obsession is fleshed out much better.
As with Two Towers extended, Eowyn's love for Aragorn is played out more as well - and they flesh out her healing and bonding to Faramir. At some points, I felt like the extended Two Towers was cobbled together - it didn't "fit" as well as the extended Fellowship did. This extended edition really meshes well - it looks like Jackson took his time and did a great job. It's a lot of small things - there are little characterization bits tossed in throughout - including more of the competition between Legolas and Gimli that was spread through the books.
Watching Gimli on the Paths of the Dead is worth the price of the CD all by itself - the added scenes in this section are really well done, and seem to be core, not added. It was a nice touch the way Legolas recited the prophecy about the paths of the dead - and how they relate to Aragorn. The wrap to that set of scenes explains how they got to the ships as well - although fans of the books already understood that bit. It's kind of cool the way Peter Jackson is the first pirate to die :)
One very nice touch - as Denethor Walks to the tombs with Faramir, Pippin looks on - and we can see a single white flower on the White Tree (again, a difference from the bok - but a nice touch with Denethor proclaiming that hope is dead. While I'm here, can I say how much I love the arrival of the Rohirrim? Theoden's speech, the music, the charge itself? It's the most uplifting part of the entire film. The battle scene with Eowyn, Merry, and Theoden runs a bit different as well - it all fits together better.
There's a nice addition after the council where they decide to march on Mordor - Aragorn takes up the Palantir to announce himself to Sauron. That's something I missed from the books. And Frodo and Sam's forced march with the orcs - the scene timing here works a lot better.
The "Mouth of Sauron" played differently as well - I think I would have preferred the way it went in the book. The form of confrontation there was more subtle, but also more satisfying, I think.
So in summary? Very good, better than the other two extended dvd's. The timing of events is better, and it "feels" more complete. Well worth the price.