I got NI Kontakt Full during the half off sale. I've always used Rapture Pro and DIM PRO. All I can say is WOW, especially when it comes to the horns.. and really everything. IT sound's SO good. I can see I will be waiting for a sale on Komplete!!
Rapture has some nice arps and synths, the pianos aren't too bad and some of the basses are OK but Kontakt is in another league. I almost didn't buy it. I am SO glad I did!!! IF you, like me, don't know... you need to find out!
 
Julien

+1000    I don't think I've had one project without Kontakt since purchasing it.   The delivered content is good - I tend to exclusively use Vintage Organs and Session Brass for those instrument tracks, and also quite a few others along the way.   But the biggest thing for is the pure quantity of available libraries, many of them free or very inexpensive.    It's like a sample playback toy store.  
 
And FWIW, I would certainly recommend Komplete too for sheer variety.
 

And you're still just exploring the factory content, which is widely regarded as being rather weak. Many Kontakt users never even use it, because for every category of instrument in there there are third-party alternatives that are vastly superior. Get your wallet ready.

Groove3 has a very good video on it.  Even though the video is based on Kontakt 4 almost all of it is relevant to K5. January is a good time to get a month for $15 and hunker down and stay warm and learn some stuff too. Have fun.

It's worth doing a course or reading the manual on how to create your own instruments too, because it's fairly easy and you can make use of those raw sample sets you may have lying around.

Hmmm... that is something I've never done.    Thanks, Sharke, there goes a few (hundred) hours.  

Enjoy schweet Kontakt, Julien!

I know there are easier ways to load samples onto keys (heck you can even do it with Session Drummer) but Kontakt is unbelievably flexible and powerful when you look into the key mapping, sample grouping and the extensive effect processing. 

I have been trying to dig out the thread with lists of free and low cost libraries but its proving hard to find.
 
Here are a few off the top of my head. At the very least it will give you an idea of the breath of stuff available.
Hollow Sun - the Novachord stuff in NC346 is of particular note.
Bolder Sounds - lots of off the wall stuff. Music Maker v4 is probably the highlight here, but the Tibetian Bowles are alos very good.
Soniccouture - I love their RMI Rocksichord, the concert Kazoos may be a bit of an acquired taste.
Analogue Drums - Big Mono is a great drum kit!
Soni Musicae - for Harpsichords, bells, acordian and a rather wonderful house organ.