I have been looking forward to this ever since booking our accommodations in Interlaken although I knew it was weather-dependent. In the last week I was reading the most recent reviews and was a bit disappointed to see some more negative ones talking about how crowded it has been getting at the top at peak times. We read a bit further and it looked like going early was the way to go, and the weather looked pretty certain to be good today so we bit the bullet, bought tickets and seat reservations and set off from Interlaken West before 7:30am (we were on the train that left Kleine Scheidegg at just after 9am and returned from Jungfraujoch to KS at 12:13pm).
We all loved it. First of all, yes, the Sphinx platform was a bit crowded, even early. It snowed yesterday so half of the platform was closed. However, the big tour groups don't seem to go out to the snow play area/ walk to the hut. You don't have to walk all of the way to the hut to get out away from people. As far as the restrooms go, since the trains arrive every half an hour, try to either get in front of the big tour groups and use the bathroom right away, or else lag behind them before the next train arrives. There's a bathroom at the Kleine Scheidegg station as well so if you have seat reservations you'll have time to use it there before transferring (I think.. not sure if that's true for every arrival).
We took a small snack and water bottles and then had lunch down in Kleine Scheidegg. BTW there's another restaurant there besides the crowded one at the train station and the expensive one in the Bellevue hotel.... go over the train tracks and up toward the cute cows.