Serious photographers don't waste time on forums.I'm sure there is a great deal of truth in that. Still, forums are the new meeting place for photographers since the days of going to the lab several times a week become a distant memory. That was a great place to have quick conversations with other photographers. It can be quite an isolated profession.
But that's getting off-topic. I've kept my 24-105 f/4is after getting the amazing 24-70 f/2.8II. There are occasions where the IS is valuable, for events work the extra reach is valuable and it makes a great travel lens. I prefer to travel light and generally go with one body, a de-gripped 5D3 and the 24-105. After working all year with a dozen high class lenses within reach, vacationing with just the 24-105 is creatively stimulating as you are constantly pushed to make new decisions about how you're going to approach a subject. It's fun. Some hard working, busy photographers will travel with just a body and a 35 or a 50 prime, maybe with a Leica....or a Sony RX1!
$800 for a pre-owned 24-105 is a very high price. You may want to take it. But I'm certainly keeping mine.