I rely on Kontakt every day - I also agree that the default factory library sounds are good, but within any given category they are pretty limited.
There are libraries available for free and some incredible others that can cost quite a bit.
Even Native Instruments releases Kontakt instruments/sounds - like their new Brass expansion (cannot recall exact name). I did listen to the demo, and it DOES sound mighty.
Dim Pro is the included free sampler provided with Sonar (not sure what comes in with anything other than Platinum, but Dim Pro certainly does).
some of the Dim Pro sounds are quite usable, and indeed I just literally yesterday afternoon replaced a solo violin I had been using Kontakt 5's factory content for, with a violin patch from one of the included expansion packs for Dim Pro that I had - from the Garritan Pocket Orchestra. It actually sounded much crisper and less reverb on the recorded samples themselves (with no effects - conversely, the Kontakt's factory content violin had too much reverb even with all effects turned off).
So, you will have to listen and decide for yourself - I ended up with both, and use Dim Pro for some sounds, and Kontakt for others. I look at potentially picking up additional Kontakt libraries as needed for whatever projects I have coming up, or if I come across one that just sounds great enough to know that I want to replace/augment my existing libraries). I do not seem to ever need to go seek out expansion packs for Dim Pro, because there are really really good libraries available for Kontakt and Kontakt IS indeed the king daddy sampler available on the market.
Bob Bone