Just to give you fair warning, I ramble a little bit in this response.
The default rules, in my opinion, fall a little bit short. I have uploaded my own rules and instrument default files to the forum that better utilize GPO's legato features as well as a few other less common techniques. I actually have update my rules slightly (and have a few more ideas), so I'll post an update in the not too distant future (as time allows). The rules in their current form can be found here:
viewtopic.php?f=24&t=1285In terms of the sound quality of GPO vs. Notion:
1. I think that GPO's strings (apart from the harps) are the weakest part of the library (the harps sound great; I think I read somewhere that Gary Garritan was or is a harpist, but I'm not sure). Notion's strings (especially with the expansion sounds) go a lot further than GPO. I still use GPO, but I like to layer the solo and section sounds underneath Notion in the mix (to give the strings a little more depth).
2. I think GPO's woodwinds are some of the best sounding woodwind samples out there, and I tend to use them first and foremost. Most GPO woodwinds have multiple solo woodwind and brass files, so layering these sounds between woodwinds pairs usually gives the most cohesive sound. You can certainly combine these with Notion's woodwinds with good results, but I just like GPO's woodwinds (this, I think, is completely subjective).
3. I think GPO's brass is pretty good; however, it takes a bit of experimentation and practice. The best results come from mixing the GPO solo brass patches with the GPO overlay patches. I tend to layer these with each part (that is, I assign an overlay part in the same Aria instance and give it the same MIDI channel as the solo patch). This works pretty well; however, there are certain expressions that work better if you utilize the overlay patches via a sequencer staff separate from the notated part; this is beneficial because you can manipulate the sequencer staff separately and uniquely compared to the main instrument staff (let me know if that makes sense; it can be a weird concept/technique to learn at the beginning).
4. The rules method/syntax employed by Notion don't really support GPO's percussion. I can use GPO's snare and bass drum as well as the melodic percussion without a problem, but GPO includes a cymbals patch, a general percussion patch, etc. These patches have multiple instruments and different keyswitches to trigger various techniques which are all grouped together (this will make sense as soon as you try loading these instruments in Aria). GPO is designed this way so that you can play multiple instruments live from one patch; however, because Notion only has one instrument-ID for the patch, you can't isolate one cymbals rules from another. Therefore, it's impractical, if not impossible, to fine tune the playback of these instruments via the current rules system (especially for techniques like unmeasured tremolo, alternating hits, etc), so I usually stick with Notion's percussion. Notion's percussion sound pretty good and there's not need for rules tinkering.
On the whole, I think blending the two libraries together has a very nice sound. Garritan products emphasize the technique of ensemble building (using different samples from the Garritan libraries [each with different characteristics] to form a more realistic sound). I think mixing various libraries together is the logical next step in ensemble building and, fortunately, Notion makes this process very easy to set up and adjust.
Does that help?