Dianedancer - seems you got lots of good advice already on pros/cons but one more thing I would mention from my experience is that you will enjoy it MUCH more if you can manage to go early as possible in the morning before many tour groups/others arrive. We love the region and visited three summers in a row exploring various areas - the two visits we didn't do the Jungfrau trip due to a combination of weather and wondering similar to you whether it would be worth it (plus we had young kids). We finally decided to go last summer (kids were 4 & 6) and took the first morning train up from Wengen as we were staying there. We were really happy that we went as it was a very different experience than anything else we had done in the region. First, you get to make a quick stop at a station on the way to the top and look out the side of the mountain which I found very interesting. When you get to the top my first impression was "oh boy, here is the touristy gift/watch shop" but then once we got past that and were able to go out onto the observation platform and take in a very different view of the mountain range and snowy terrain, it was quite fascinating (and very cold/windy). Then we enjoyed walking through the exhibits and the ice sculptures/caves and finally walking out on the snow where they have a small sledding hill set up. There were a number of people but it was not crowded at all. HOWEVER, by the time we finished wandering around through the exhibits and outside (which took maybe 1.5 hours in total)and came back into the main visitor center area, there were hordes more people and it quickly became much less fun and I had to keep tight control of my kids due to masses of tour groups so we were really happy to be leaving by that point. I am quite confident that if I had to sit in a crowded train all the way up and fight the crowds at the top, it would have been a far less enjoyable experience for me but admittedly, I have very low tolerance for crowds.

BTW, we took our kids and my mom (65 years) on the walk from Mannlichen to Kleine Scheidegg (KS) and we all loved it. Took us about 2 hours with a 4 year old. Although it is a busy trail, it is nice and wide and the views are glorious. It's pretty much flat/downhill towards KS until the last bit where you go up a slight incline just before you head down to KS.

The only time I was up there, a young boy threw up in the train on the way down. There was no way to escape the smell until I found some perfume in my purse and put it under my nose. It was a long ride down.

NorCal and kleeblatt - When did you go? We will be there in the third week of September. Figure there won't be many children. So maybe we would only have to worry about the adults throwing up ; )

Not sure about leaving really early as we will be with another couple and I doubt highly that they would be up for a 6:30AM trip. I wonder truly if we would!

We were there much later but the train was pretty packed. This was many years ago (8?) and we went in October. The third week of September should be a good time to be in Switzerland. The views are crystal clear, the cows are coming down from the alps and there are many local festivals going on.

Updating...having done both the Schilthorn and Jungfrau 2 years ago, my vote goes to Aiguille Du Midi near Chamonix France. DH preferred Schilthorn over Jungfrau.