If so, is it good value?

IMHO - no. I did this trip several years ago. For less money, you can rent a car and drive to the top of Twin Peaks and have a much better view of the City. Or, if you have some good walking legs and don't mind some hills, get the book "Stairway Walks of San Francisco" and explore some great neighborhoods with some different views of the City. What you will see in a few minutes in a helicopter will leave no lasting impressions.

Thanks SFFlyman.
I guess a seaplane would be more or less the same...?

If the experience of being in a seaplane (or helicopter) is important, then that would be the most significant part of justifying the investment, in my opinion. I don't know how fast the seaplane flies (I'm sure someone here may have that knowledge), but seeing the City at 100+ mph (guessing) doesn't seem like it offers anything memorable.

It's the only way to get to fly under the Golden Gate Bridge.

I'll do it, one of these days.

but only because I don't have the guts to to risk my licence by flying under the bridge without a waiver.

Helicopter is a thrill to some. I used to take the shuttle from Marin County to SFO which sometimes had a stop in Emeryville on the east side of the Bay. It was fun.

I second the idea of the Stairway tour on foot. You will get to see some wonderful vistas and to choose many different neighborhoods in this small city.