Well I don't think the complete collection is worth it, but if you can get it cheaper then buying the individual seasons, I'd say go for it. Just make sure you can return it. The discs are sometimes scratched or finger printed (its greasy finger markings). The complete collection book, is nothing special that you probably haven't read somewhere on the net already. The bonus disc wasn't that great. I've seen better, but i've seen worse. Most of the clips can be found on youtube. The box is neat, but the disc sleeves are not. They scratch the dvds, so I ended up putting mine in other cases.
If you have a Costco membership, they have individual seasons right now. Maybe even a sale coming up. Maybe even watch Amazon or Ebay for individual seasons. New/Sealed for about $23 each. Season 5 cheaper then that. Try the trading thread here and at dvdtalk. Someone may be trading or selling them cheaper.
HTH