The ability for me personally as an individual user is that I can see what my friends are doing, how many people they've reached thus far with their posts (and also where those users came from).
It's very easy of use too! There isn't anything in particular i dislike about it - however if you're looking at this more than once or twice then maybe consider using something else? As mentioned above there aren't any downsides really but just be aware before jumping onto Adinject whether your needs will change over time. We have used ad inject quite heavily since we started our blog last year; not only do we get likes/followers etc., which helps us build up credibility within certain niches so when someone searches Google for 'blogging tips' for example, instead of seeing thousands upon thousand of results most end-up going via social media sites like Facebook first because these.