So far the reviews of the Promote feature on Twitter are not very positive (but I only listened to 2 reviews). 2 out of 2 top reviews on YouTube did not get a good ROI from their $99/mo investment.
This guy got about 30 followers in 30 days, or about 3$ per new follower. Cons noted: lack of targeting to specific groups or specific interests, and indiscriminate promotion of posts (the platform promotes ALL posts for 30 days), including those that were like personal updates, and which looked weird in the promoted status. His conclusion: targeted ad creation gives him more flexibility to spend money only on things that really need to be promoted.
The next review was even less enthusiastic
This guys tracked his results very closely week by week and used several strategies to gain new followers. He ended up losing followers in the first week of his experiment, and then it took him the remaining 3 weeks to gain enough followers back to match the baseline. He ended up gaining a net of +2 followers in a month, or approx $50/follower.