My oldest DD and I liked Timekeeper a lot. My youngest DD didn't (too many loud hisses, etc. in the beginning) and my DH didn't like it either, mostly because there were no seats.
The "no seats" part is what I heard people complain about most.
The last times we went on it, it was far from full. Most people didn't listen to the preshow and then didn't get what was going on in the show. From what I have observed and I read somewhere, many Americans don't have a clue who Jules Verne and H. G. Wells are. Without that piece, Timekeeper is just a 3D movie with characters you have never heard of.
We saw a fair number of people walk in and go right thru to the exit when they saw it wasn't a ride. My DD and I enjoy the theming of that whole ride. It was interesting to us to see how they used the building and changed it from the 3D around America movie that was there into a "you are in the future and this is one of the new products we are showing you" theme. A lot of people are not interested in that though. We seem to hear a lot more "don't theme it at all, just drop me or move me fast" mentality in the parks. (I actually heard someone say that after coming out of TOT). IMHO, the theming is what makes the rest of the ride worthwhile and makes the trip to WDW an experience.
For 20,000 leagues, I'm surprised it stayed open as long as it did. It was slow to load and often not working at all. Even when it did work, it was a low volume ride.