depends on the track conditions. If your on a marginal track for your car & setup....yea it'll help you. If your on a track that super sticky...probably will hurt you. Either way it will slow you down a little bit pending on your angle. I'd run one as flat as you can so you dont increase drag and have a 3/4" wickerbill on the back side of the wing. Most wings will only create about 2/3's of the potential downforce for the drag it creates. If you add the wicker bill it uses more area of the bottom of the wing to create more downforce with less drag.
The car will be more stable at speeds...expecially if you go with a "mono strut" wing. The design of the strut helps to "steer" the rear of the car with the airflow going around it. Down fall as you probably know for a mono wing is the price. Consistancy of you car will be improved when on marginal tracks as stated above. Your probably not going to notice much difference on a good "normal" prepped track other than the stability. As for disadvantages to a wing....et loss (small amount), when its windy you'll notice the wind a little more but nothing to create a problem.
Wing issues aside from what it does to the car....watch your head when working around the back of the car and pending on your trailer layout....watch the trailer door and/or attached ramps.
Hope it helps you!