Alta is probably one of the best places to live in either downtown or midtown Phoenix. It's very pleasant - the staff is responsive and friendly, making sure that packages are picked up or that maintenance is done promptly. There are a lot of nice people in the building ranging from ASU students to professionals. Amenities are plentiful - a well maintained and pleasant gym, a pool table, a lounge, and a really nice pool with a small spa. The apartments are large and well designed, and an easy walk to the movie theater, Arizona center, and the light rail.
The only repeated problem I've had here has to do with noise. The building is meant to be "urban" with exposed concrete walls. Those are nice - but they also radiate sound, so if anybody is playing their stereo too loud, or if the building is having one of their Friday night parties (which range from fun to annoying depending on your point of view) the sound echoes across the walls. The management is responsive if there are noise complaints, but they can be irritating if you're trying to sleep on, say, a Friday night, or have to do some work midweek.
Still, if you want to live in downtown Phoenix, this is definitely the place to be.