I have it ingrained in me now to turn "Omni" off on new MIDI tracks I insert. I really wish there was a true "none" option. 
 
As an aside, it's worth noting that Geist is not just for drums. It's great for chopping up any kind of samples, pitching them up and down and/or time stretching them. You can have lots of fun with bass lines, piano samples, sound effects, anything. I took a piano sample from the included Cakewalk content today and chopped it up into phrases then pitch adjusted them to match the key of my song. It's just so quick and easy. 

Thank you very much Scook and Sharke for these precisions !! I certainly was a little bit confused with all the midi input/output combinations. The fact is it was more comfortable for me to have "omni" enabled because I had to use at least 2 controllers at the same time (one for playing a melodic instrument and another for its keyswitches, while listening the Geist groove)...
Scook (excuse me for my ignorance) but the "Enable MIDI Output" is a default Sonar X3 function or is a proper Geist automatic configuration , because I didn't notice this kind of problem with the other soft synths ?
 
Sharke : I have to deepen all the possibilities of Geist; now I'm just creating small rhythmic patterns! I hope that there are videos showing what you described about the melodic samples. In fact, is it to create realistic sophisticated arpeggios or "crazy" things like you can do for example with Egoist from Sugar Bytes ?
 
Just another questions which are rather worrying since it's related to system stability :
- Is Geist very CPU demanding or very light ? Is it reliable on your systems ?
- Last time, I tested very quickly the different mix presets of Geist, passing from one to another very quickly, and it crashed Sonar, ( there were Fxpansion crash report files that had been created automatically !)
- Now, I'm working on a project, so testing the Geist demo (with every 15 minutes, the Geist demo warning message that sucks !), maybe adding some Midi controller and everything was ok; I made a (long) rest without shutting down or touch anything, and when come back, I had Audio Engine drop outs every second continuously (with the well known message : the audio engine has stopped suddenly...) Given everything worked fine before and it was just a trial project, I closed Sonar and I could work normally. Weird, no ? I don't know if it this is directly related to Geist or its demo version, but if you have any idea or experimented similat things ? (I'm going to make a thread in the Sonar X3 section  because these "little" problems concern me anyway !)
 
Thanks again

As of X3 any synth that generates MIDI data has "Enable MIDI output" set on by default. Not all synths generate MIDI data. Geist may be the first synth you have used which generates MIDI data.
 
Stability issues after long pauses suggest power settings may need to be reviewed. The machine should be configured to stay powered up all the time, drives and ports should not power down. Also check for background processes and stop them.

I know I had a few crashes with Geist when I was using the demo version - perhaps they haven't updated it. I'm pretty sure there's been a Geist update since I installed the full version. It certainly never crashes my system now. I sometimes have 2 or 3 instances of Geist, each with all 8 engines running (i.e. 8 groups of 16 sample pads, each with their own effects chains, then effects chains on each engine as a whole etc). It seems pretty efficient. 
 
Don't forget you can also route the Geist engines to their own audio outputs so that you can use your own effects. Sometimes I like to process each pad with its own effects in Geist, but then I send the whole group of 16 pads (an engine) to its own audio output so that I can apply my own compressors, distortion etc. 
 
I don't know what you mean about creating sophisticated arpeggios, Geist isn't really a synth, it's just a very powerful sampler/drum machine. What I meant about using melodic samples is that you can work with them just like drum samples, slicing them up and rearranging them and triggering them from pads etc. You can edit the start/end points of each sample, pitch them up and down, time stretch them, apply envelopes etc. 
 
This is a nice video which shows a typical workflow in slicing beats, he also slices a synth pad part and some bass as well. 
 

Again a very big thank you for your answers and help !!

There have not been many updates since i purchased, but they did release a beta to owners that resolves some of the issues some had with spitter.

Ok Dub; thanks !

sharke
I have it ingrained in me now to turn "Omni" off on new MIDI tracks I insert...

+1... MIDI Omni is your worst enemy! MIDI Omni is the "default" for everything... and any active synth will play what it receives (MIDI controller, synths with MIDI outs enabled, etc.). Develop a habit of assigning channels to MIDI components immediately when inserting things (and keep track of them).

peter434
- Is Geist very CPU demanding or very light ? Is it reliable on your systems ?

Geist "can" be very intensive on a CPU, so is good practice to "minimize overhead." A "clip" is essentially a "window into a wav file, and it seems Geist loads the entire file to create that window. Effects/processing add to this, so I have gotten into the habit of only feeding Geist "what it needs." If I want only 1 second of a wav file, I chop the original first, and give Geist only that 1 second. It is entirely possible for Geist to load 100MB and play only 5 seconds of it all. Just keep that in mind. I am NOT sure how Geist does its internal processing, but if you feed it a lot, it can easily consume a lot of system resources.
 
It is generally good practice to think of this with any VST/effect. If you have done what you need to do, commit it to a wav file... otherwise SONAR is "processing data" on each run (complex) versus simply playing a wav file back to you (incredibly simple).

Thanks mettelus !
Concerning Midi routing, but, maybe I'm wrong, it seems that even if I specify one specific midi channel input in other soft programs, Geist still is triggering them and all midi programs , whereas it's routed to nothing; So, as Scook suggested, it's necessary to unchek its "Enable MIDI output" because, indeed, Geist may be the first synth I have used which generates overall MIDI data.