I've been wanting to try Alta since they opened, and I am so glad that I finally made the time to go; I was so impressed by their food that I went back twice the same week!
It's better to make a reservation no matter when you want to go because chances are there will be a wait. Note that their reservation system will charge your card $45 for a no show if you do not change or cancel your reservation more than 24 hours in advance.
The main outdoor patio is my preferred seating area, but to be avoided on the rare rainy days in Los Angeles. Also to be avoided, the tables by the right rear ramp, which is unpleasantly close to odors emanating from the back alley :(