Pros:
-Faceroll early/mid game
-Dominate your lane (get TONS of CS while ruining your opponents')
-Deal tons of damage early/mid game while still being tanky
-Core items come in managable parts
-Very strong lane presence and pusher
-Still be an extremely strong support healer and disabler for your team

Cons:
-You fall off in super-late-game (45+ mins, you simply become just a somewhat gimped support)
-Not as inherently bursty or mobile as other AP mid champions

Doing your job as AP Mid
Arguably the most important aspect of the AP mid position is to shut down their opponent and establish control of early/mid game. This is done by establishing strong map control (defending/destroying towers, keeping lanes pushed, etc), and ganking and denying carries. Soraka does this extremely well. This build is extremely tanky, push heavy, and does tons of damage early on. You will dominate.

Lane Phase (1-10 mins):
Spam Starcall to push the lane, get TONS of CS, harass your opponent, while ruining his cs (push the creeps into the enemy tower) and eventually kill their tower.

Early-Mid game (10-20 mins):
Try to setup a gank with your massive Starcall damage in top/bot lanes. Be sure to apply as much pressure as possible particularly on their ADC from this point forward. Begin pressuring the other teir 1 towers if they are not down yet.

Mid Game (20-30 mins)
You should be pretty tanky and have your heal maxed. You should have taken all the teir twos by the end of this phase and should be pressuring the inhibitors.

Late Game (30+ Mins)
Hopefully by now you should be at the enemies doorsteps. You should now be focused completely on keeping your team alive and preforming the standard duties of a support Soraka while still dealing lots of damage and maintaining a healthy mana pool. Victory should be in sight.

Laning with Starcall is extremely easy. It does tons of damage, its easy to CS with, has a very low cooldown, and is easy on the mana pool, especially with all the mana regen from runes and items. You will find yourself tripling your opponents CS, doing more damage to his turret, and quickly blowing through all of his consumables, sending him back quickly. You are also raping his CS by pushing the creeps to the tower. With a little skill, you can harass your opponent even when he is hugging the tower with Starcall and Infuse. You will utterly deny mid and rape it without serious support from either a jungler or a serious lane composition change from the opposing team, which takes time and coordination. Please remember remember that you have a free nuke in your Infuse ability to help harass.