The online demos absolutely influence my potential decison to purchase. If the thing sounds bad in a demo then i am not going to waste my time digging any further. If it sounds good in a demo then i may investigate further. I never buy just because of a good demo. But if i never get to hear the thing at all because

a, Music shops dont stock arrangers
b, the instrument in question is not sold in my locality

Then go to forums like this and listen to opinions and ask questions but i treat the responses very carefully as everyone has their own pet bias.