We still use ours for our son at age 27 mos as a booster chair at my mom's house. Most kids probably won't get that kind of use out of it, but if your little one has skinny thighs, it works for a long time. If they have chunky thighs, you might not get your money's worth out of it. At first I thought I had wasted my money on it, because my son didn't like to be in it for very long, But he would stay in it for a while. Which was nice because he pretty much fussed if I wasn't holding him constantly. I have the tray also. It is kind of awkward to put it on because you have to lift the bumbo with baby in it. I wouldn't leave my son in it for too long to be on the safe side with their developing spines, but my son didn't seem to have any problems learning to sit on his own, he has always been a little early to those kind of milestones.
Please DO NOT put baby in the bumbo up on a surface like counter or a toilet like another poster mentioned! They are not that stable. My son would lean over and tip his over on purpose. It isn't hard to do. I can't imagine how dangerous it would be to set it on a toilet and have them tip off the toilet onto a tile floor. I could easily see it snapping their necks or cracking their head open. It would be much worse than a regular fall off a high surface, because their body is immobile in this thing that gives extra weight on the fall.