I have Sonar X3, along with Melodyne  and Native Instruments Komplete...what would I gain by upgrading
to Sonar Platinum? Is it worth the upgrade?
Thanks for your help!

Go to the http://www.cakewalk.com/Products
And look over the features. Only you will know what you will use, everyone is different. 
 
X3 is a very good version so don't toss it out. I still use mine on one computer and I don't reall feel there was a big differance other than mix recall etc.  I upgraded just to stay on top of progress and because the offeres are super reasonable for what you get with it. Even though I've yet to use 90% of the features it's good to know they are there. I don't upgrade every month as my DAW is off line. I'm 2 behind right now. 

This page has the complete feature list for the various Sonar 2015 versions...
 
http://www.cakewalk.com/Products/SONAR/Versions#start
 
There's really too much new stuff to really list easily or know what might appeal to you.
 
There are improvements to certain areas of the program too not really covered in the list like...
 
The new Control Bar (better module management like collapse/minimize/flyout (of minimized mods)/left right scrolling... there is also a new Custom Module where you can program commands you use frequently to buttons and I think you can map the buttons to an external controller)
 
Mix Recall (let's you save and recall mix settings... this has a new Control Bar module too)
 
FX Bins are now FX Racks (basically the same thing but a little prettier and the rack expands so all modules are visible instead of scrolling through insert effects)
 
Virtual Keyboard (let's you control soft synths like you would with an external controller except on screen or with your QWERTY keyboard)
 
Audiosnap and Staff View have gotten some fixes/improvements
 
There is a new "Paint" tool for copying and "Painting MIDI
 
PRV has been improved and (IMO) is easier to view/work with due to larger notes. You can also "time stretch" MIDI clips now like the audio Time Stretch feature.
 
There is now "track to track" routing so can essentially create busses in the track pane now like in other DAWs. You can also record into those tracks (including synths) so that's a pretty big deal.
 
That's the stuff that might not appear in those feature lists. There rest is a buttload of new tools, plugs and whatnot (on the lists).
 
It's basically X3 but with a bunch of new stuff. Lots of fixes too.
 
Do you NEED it compared to X3? Probably not because X3 was pretty good but there sure is a lot of cool stuff now that has made my work a lot easier/more fun.
 
Cheers.

X3 is solid, but I found the upgrade very worthwhile and a good value.

There are improvements to the user interface. I like the start screen, there are smaller improvements that help more than one might imagine. And some address problems I never had. As Beep recommends, check the list. YMMV.
 
You could buy your own 3rd party drum replacement tool. There are several good, mature products out there.
 
But bugs that I had in X3e that slow my specific workflow are still there.
You might find that bugs which slow your workflow are fixed, the bugs-fixed-per-version info is freely available somewhere.
 
Annoyingly, newly introduced features overlap with newly discovered bugs. I have spent many hours nailing down steps how to reproduce them, only to post them and never get an acknowledgement.
 
I decided on a one-year payment.
My next payment will depend on the state of bugs when this year's payment expires.

If you DO decide to upgrade (well worth it IMHO), do NOT uninstall any previous version(s)
The footprint is small and you stand to lose shared plugins if you remove earlier versions

jpetersen
 But bugs that I had in X3e that slow my specific workflow are still there.
 

To the OP...
 
I've had at least a dozen "bugs" (or at least annoyances) that used to affect me regularly (or semi regurlarly) disappear with the rolling updates/fixes.
 
You can see all the bug fixes that have occurred in each update in the monthly eZines.
 
The eZines can be found/downloaded (pdf) from this page...
 
http://www.cakewalk.com/Products/SONAR/Rolling-Updates#start
 
Just scroll to the individual update listings and click the "Download the eZine PDF" link for each entry. Depending on your browser settings they will likely just open in a new browser window without actually downloading to disk.
 
I personally download them all and keep them in the folder with my manuals so I can reference them as need be (they include basic instructions on how to use new features as well as the list of fixes).
 
Note: Some of the fixes that get listed are actually things that got broken with newer updates but many (if not most) are fixes for older, long standing problems. Seems we get maybe one or two new (and somewhat obscure, system dependent bugs) every month but at least a dozen fixes for older stuff.
 
I haven't really encountered any "new" bugs myself.

I too recommend studying the Sonar Plat feature list and seeing if there's stuff there that you really need/want. 2015 didn't offer anything of import for me and I would have been happy to sit it out. 

Absolutely! Just do it! It's awesome!

OP
For what it's worth, you may have to ask yourself a few questions about why YOU would want to upgrade.
 
For the most part, X3 is solid and many here would attest to that. 
My suggestion? If you aren't looking for new features or a fix to a bug - wait.
 
Are you an early adopter type?  If not, you might wanna wait as well. 
The updates to Sonar Platinum come monthly and quite a few updates have had new features that will take longer than a month to learn and utilize skillfully.  If you're one that just wants your platform to be static for a while, SP will have to wait.  But also, keep in mind you DON'T have to update it EVERY month.  That will be totally up to you as you see fit. 
 
So is the price of the upgrade worth it?  ABSOLUTELY!  And eventually you'll wanna upgrade.  Just know what you're in for when you do and you'll be a LOT happier for it.