Very good...could be great.
We stayed at The Westin San Diego (Emerald Plaza) for four nights in August 2012. The room was a good size, though a bit dated, and the bed was very comfortable. We accepted the "Green Option", which if I recall correctly, meant that our towels would not be changed for the first three days. In return, we received $15 ($5/day) in vouchers to use within the hotel. We were also pleased to see that our complimentary bottled water was replenished each day. 
The downtown location was very convenient. We did not have a rental car for our stay, and we were able to easily get to the San Diego Zoo, Sea World, and Old Town using public transit. It is also walking distance to the Amtrak train station/Trolley station, the Harbor, Little Italy, and Horton Plaza. The hotel provides a complimentary shuttle to the airport. There are a couple minor drawbacks to being downtown - there is a homeless population as with other cities (they did not bother us) and the proximity to the train station meant a bit of extra noise.
There were a few things that prevent me from giving this hotel the highest rating. 
Service: We used our vouchers for Starbucks coffee at the Aroma Cafe. The woman who served us twice during the week was unfriendly both times. She said the right words ("My pleasure."), but it felt very insincere - the lack of eye contact, no smile and tone of voice were a stronger message than her words. 
Value: We were annoyed to find an invoice for $3.00 slipped under the door on the morning of our departure. We determined that it was for two phone calls to a toll free number. It's not a big amount, but it left us with a negative feeling after a nice stay. Perhaps business guests whose bills are forwarded to their company's accounting department don't notice (or care) about these charges, but we felt "nickeled and dimed" for lack of a better term. Note that this hotel has an extra charge for in-room Wi-Fi ($12.95?), which is frequently offered at no additional cost at other hotels. We chose to use the free Wi-Fi in the lobby.
To end this review on a positive note, the front desk staff and the airport shuttle driver were all friendly and helpful, and we enjoyed our time in San Diego!