I also did the Grosse Scheidegg loop a couple of weeks ago. We started in Lauterbrunnen and travelled in the opposite direction, climbing Grosse Scheidegg from Grindelwald. The climb is tough because it averages more than 10% for the last 4 kms, you'll certainly need some LOW gearing (see the profiles on climbbybike.com for coming from
Meirngen or
Grindelwald). The scenery was certainly pretty awesome. When the buses and service vehicles approached (no regular private cars are allowed on the top section), we always got off of the road to let them pass because the road is really narrow. To get to the base of the climb, we stayed on the main road into Grindelwald. The road is pretty busy and there is not always much room for the traffic, so its certainly not a great part of the ride. However, I knew the alternatives would involve gravel tracks and lots of energy-sapping unnecessary undulations, so we put up with it because we knew it would improve after Grindelwald.
After all that climbing, we were ready to be rewarded with a great descent. We knew that we'd have to take it careful due to the road width, quality, and possibility of meeting a bus head-on. However, the descent was FAR less enjoyable than we'd hoped. The road surface was pretty terrible, and would have been more suited to something a bit more cushy than a road/racing bike. We hoped that after the point where the road was open to cars again, it would improve. However, the change was imperceptible and it was still narrow and bad quality, the only difference was that there were now more vehicles to worry about, making it really difficult to enjoy the descent.
We stopped in Brienz, where we found some awesome cake at a Tea Room / Bakery on the west side of town, then continued along the north side of Lake Brienz. This is a beautiful ride along a well-paved road. There is a moderate amount of traffic, but the road is plenty wide enough. There are no significant climbs IIRC. I've now heard a couple of people complain about the ride along the south shore of Lake Brienz, and so I'd recommend everyone to use the north shore unless they want to throw in some additional adventure to their ride.
We then skillfully managed to bypass nearly all of Interlaken and rode straight back to where we were staying in Lauterbrunnen - a section in which you ride straight toward the Jungfrau and is a great way to end such a scenic ride. Another quick note, if you do the ride on a weekday then there is a good chance that the Swiss Air Force will be flying their fighter jets in and out of Meiringen, and you might get to watch many of them fly around throughout your ride, as was the case for us.