Greg,
The Jaco demo impressed me a great deal though, and after reviewing other fretless bass guitar options from other vendors, I really couldn't find anything that matches it in detail, execution, and especially fluidity and phrasing. If this library ends up getting picked by the KVR members (which I am not one of and have no intention of becoming due to the high percentage of trolls on that forum), as the sale item for your holiday promotional, you can be sure that I will purchase it.
I am sorry, but Pettinhouse wins my vote for electric guitar playability and believability, and of course I also find VSL's excellent 7-string electric guitar to be invaluable for contexts that need that pre-overdriven sound (for which it is intended, as that is the way electric guitar is usually used in modern orchestral compositions).
At any rate, it is all relative, as there are many good choices these days -- especially compared to even five years ago. Phrasing matters to me way more than timbre, as the latter can at least be modified somewhat via filters. Not everyone has this focus on their library evaluation, and this is one of the reasons I am in the minority when it comes to being in love with Pianoteq (which most people seem to hate).