Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I specialize in support classes, and I've picked up Sona as a pet favorite lately. I do very well with her, people tend to tell me my Sona is very good, and I feel she's a very good asset to the team (as much as Morgana in terms of utility and dps). I hope that this build will be able to help anyone who wants to pick up Sona.
The Mejai's Soulstealer is an important core item for her, because it's so easy to build up to 20 stacks (either kills or assists), and it gives her godly heals and good burst damage. Any AP item after that is fluff. Of course, the important part being not dying. I usually end games with either no deaths or less than 3 deaths.
Movement speed quints, ghost and flash are essential for Sona IMO. She has NO escapability and building excessive tank items on her are just a waste of gold. In fact, I'm considering switching the last health quintessence for another movement speed quint. She doesn't have Morgana's Shield/Snare combo, nor Janna's Shield/Tornado/Ult combo, or even Taric's stun-n-run option; she only has her ult, which has a long cooldown, so mobility is very important to her.