We went with our teenage boys for 4 nights in April.
The weather held out for us, sunny every day, and we had plenty to do in Beatenberg including hiking, mountain biking (recommend renting at Eiger Sport in town), riding the funicular down to the Thunersee and exploring the St. Beatus Caves. We also loved going into beautiful Interlaken for long walks and some great restaurant choices.
The apartment is really very good quality. The kitchen had great appliances and we loved the espresso machine. The primary bathroom was especially nice with the rain shower and big soaking tub. The patio is huge, great for having meals with a million dollar view.
The owner was very nice and helpful. It would be helpful to travelers to practice your German, but even if you don't Speak German, as in our case, you are able to communicate with out too much trouble.
I would highly recommend this flat to anyone who wants to enjoy the natural beauty of Switzerland.