The first two disks (Walking with Dinosaurs, The Ballad of Big Al) are fantastic. They immerse you in the prehistoric world and make you follow the adventures of various animals. It's incredibly well done, and very emotional. The best nature show I've ever watched.
The third disk (Land of giants, The giant claw) is much weaker. No immersion, no story, no emotion. An annoying guy traveling with his car or small plane among the prehistoric animals. This of course ruins the whole effect. Showing bones ruins the effect too. They should have put the CGI in one show, with a little story to keep things flowing nicely. They they should have put Nigel and the bones in a "making of" extra.
Anyway, don't let this stop you from buying the big box. The first two disks are totally worth the price by themselves.