There's pretty much nothing to say about iDVD. After all, how much can you update a DVD writing application? There's a bunch of fancy menus, themes and so forth, but fundamentally it's just more of the same this revision.
As with most application suites, it's unlikely you'll buy iLife with the intention of using every program available. Luckily each - the value-add iDVD aside - feels like it is worth the £70-odd asking price of the whole package. Presumably Apple's marketing folks are smart enough to know trying to charge 'full price' for each application in the suite would be a losing battle.
iPhoto is a great basic photo editing suite, iMovie is an excellent tool for putting together slick home movies easily, GarageBand is as great as ever and Artist Lessons are just inspired, and iWeb is, while somewhat restrictive, still a fantastic bit of kit for simple website creation by those not interested in learning any actual web coding. Oh, and iDVD... well, it works, so that's nice.
iLife '09 may be just an evolutionary update form last year's version, but that evolution is far from incremental. If you don't need the new features, don't rush to upgrade, but if you've been holding off a purchase pending a better application suite coming along then you should wait no longer.