I installed t make sure there were no system issues, before buying Samplemodelling brass. Then soniccouture had a freebie organ available. Their stuff runs on the Free Player, but they give you a selection of free sounds with each purchase. These free sounds require the full player. 
 
http://www.soniccouture.com/en/products/
 
 

Yes the free Kontakt 5 player is worth installing.  It's a good way to gain exposure to how the full version of Kontakt works.
 
Run a web search for Kontakt free and you'll find many more soft synths are available using the free player.  Many developers will include a link to the free player on their website.   Some even have an arrangement with Native Instruments that the cost of their soft synth is deducted from the cost of the full version should you decide to upgrade down the road.  Here is one such website:  http://realitone.com/

The third part instruments I am talking about are fully paid for full versions of these things. Not freebies. Freebies are generally pretty limited anyway. There is no restriction though in terms of that 15 minute time limit.
 
Some other third party stuff will work in the player but as jmasno5 pointed out they will be limited to the 15 minute time limit.
 
Other third party synths just won't work in the player at all and you need the full version of Kontakt.  (which I feel is a bit of a bind because it is quite a serious outlay just to get another instrument to work) I generally avoid those all together. 
 
Anyway it is no harm to install the player.  It does not effect your system in anyway and I use it mainly to use other 3rd party fully paid versions.  (plus a few Native synths too) There are still quite a lot of them out there that work perfectly in just the player and with no time limit.
 
You do have to check though. Especially before buying a big expensive synth. Often it is in the very fine print and you have to go looking for it too. (e.g. requires full version of Kontakt)

fjz
I'm gonna do a little more searching for some 3rd party free libs that will work with the "player" version.  So far though, I have not had much luck finding any.  If anyone knows of any, please do share.  I did also have a quick look at the Heavyocity offerings (mentioned above), but the only freebie there appears to be some loops.

You are not likely to find many because NI charges developers a steep fee to allow their libraries to be used with the player.

I believe most free sound libraries for Kontakt will require the full version of Kontakt.
 
However there are many paid libraries, both from NI and 3rd party, that will run on the Kontakt Player.
 
The Kontakt factory selection that you get included for free with Player is a sample of instruments in the full factory library.  Band, Synth, Urban Beats, Vintage, and World are the included categories.
 
The 'Band' collection has some nice basses, guitars, an E-piano, organ, etc.  Good enough to get a feel for the plugin.

The bagpipes are worth having in Kontakt Player though the duduk is a bit disappointing. Well they are for me as I do folk influenced stuff

Magic Russ
You are not likely to find many because NI charges developers a steep fee to allow their libraries to be used with the player.

Ah, that explains a lot.  Makes sense.

JoeGBradford
The bagpipes are worth having in Kontakt Player though the duduk is a bit disappointing. Well they are for me as I do folk influenced stuff

When I read your post, I remembered that I had seen a duduk freebie posted somewhere (probably here).  I searched it out and found this (and it states that it's "made for Kontakt Player" in the description).
 
https://www.strezov-sampling.com/products/view/BALKAN%20Ethnic%20Orchestra%20Duduk%20Freebie.html?utm_content=buffer3cbbd&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer
 

michael diemer
 It was quite bewildering at first how to actually load the library and then load an instrument. Took me days.

yep, me too, but for a different third party library, though I would recommend the OP give it a shot; if you are like me you already have eleventy other vst instruments that collect digital dust, and I will focus some of myself on those too.
 
JQS
 
 

Thanks again for your input, everyone!
 
I've just installed everything, and checked out most of the patches.  There's definitely some stuff I'll be able to use in there. 
 

Jeff Evans
Some other third party stuff will work in the player but as jmasno5 pointed out they will be limited to the 15 minute time limit.

I have a couple of questions regarding this 15 minute time limit thing, if I may:
 
1 - Is that 15 min limit only for the current session I'm in... meaning if I close out, reopen and reload, does the 15 minutes start up again?  Or, is it 15 minutes total and that it?
 
2 - I'm not clear on if this 15 min limit is to allow you to demo paid-for libraries before purchasing.  Is that the case?  Or is it to demo libs meant for the "full" version of Kontakt?
 
fj