The Millennium Tower is for the person who sees value in services and amenities in addition to having a nice place to live. If you intend to take full advantage of having a wine-tasting room, club room, swimming pool, gym, on-site celebrity-chef restaurant, wine locker, pilates studio, etc, then the Millennium is a premium choice in the SF marketplace. Imagine having your dry cleaning sent up to your unit while you slide the concierge a tip for making your tee-time reservation as you walk out the door to your car, which the valet brought around for you. That's what this building is all about... luxury, service, and amenities.
The location is good and SF's central point is tipping towards South of Market. However it will take many years for SOMA's vision to fully manifest itself, with the development of many new projects and the Transbay Terminal. If you're buying into this location because it will be the new center of SF, be prepared to wait a while. The nice thing is that it sits on the border of a well-established area and an up-and-coming area, so you kind of get the best of both worlds... as opposed to One Rincon Hill which is out in my opinion is out in no-man's-land for the forseeable future.
The units are nice but not earth-shattering. The prices are among the highest for SF's highrises, but this should come as no surprise as the building is chock full of amenities, services, and marketed as uber-luxury. They recently had an across the board 15% price drop on all units (including those already in contract, which was a classy move). This spurred sales a bit but they have a ways to go. The sales center says that they do not negotiate. I know agents that have tried to negotiate with them anyway and have gotten nowhere. So there does seem to be some merit to this claim. If I were representing you I'd still try anyway. Developers of massive buildings like this one definitely do not have "the power" right now. Buyers do.
Since you're seriously considering this tower, I would ask myself these two questions:
1) Am I going to own long enough to benefit from owning here?
2) Am I going to take advantage of all of the amenities they offer? You certainly pay for it with your HOA dues, so not using them is throwing money away.
Hope this helps!