This is a grand old, venerable classic in the middle of the Union Square shopping district and right on the Powell Street Cable Car line. The lobby is always abuzz with interesting, typically wealthy-looking beautiful people, arms laden with shopping bags. A place to see and be seen.
Unless you're paying for a top-end suite, you simply MUST insist on a room in the "new" tower, not the old side of the hotel. While the old hotel looks far more charming, the rooms are small and extremely musty and should be avoided at all costs. Fight for your rights.
Also, if you're a Starwood Preferred member, do not expect to get any preferential treatment if you sign up for this hotel through Expedia or another intermediary. The hotel staff will give you no special service (e.g. room upgrades). Indeed, you're not even allowed to stand in the shorter, "Gold Preferred" line up at reception.
In the tower, the rooms are huge but comfortable. Very tastefully decorated, this Westin almost has the flair of a W but for a lower cost.
And don't forget to have a drink in the lobby bar. A unique, old-world experience.