We've stayed here several times now, both while we lived in SF (transitioning between homes) and now as tourists. It's a splendid hotel with exemplary service, gorgeous accommodations and a solid location.
There are lots of 5-star hotels to choose from in San Francisco and while in many ways they are comparable, each had distinguishing characteristics like the views, location, etc. For the St Regis, it's really the service that stands out. Here, the customer is always right and they make you feel like a VIP at every step in the process.
Room service is exceptional and while you're clearly paying exorbitant 5 star prices (like $10 for a glass of apple juice, and that's before the tax and service charges); the details, presentation and follow through make it generally worthwhile. They have a ton of kids menu options, BTW.
The lobby is stunning, opening up into the bar and lounge areas with warm light and modern finishes.
The entrance while happily visible from 3rd St is still tucked away, so you're removed a bit from the less desirable elements of SOMA.