I've probably bought 99% of my Steem. I'm far more interested in getting "my stuff" adopted than any monetary return, which of course requires people to read it at a first step. As a noob I thought it would be a good idea to have over 1000 SP, so bought that. It made no obvious difference except for a few pennies here and there. I was curious how some people who posted rubbish had reps of over 70 (not you tarazkp!). After 7 weeks of careful posting and commenting, probably a few hours a day, my rep was 35. It seemed that pretty much everybody had a rep of 60+, and that was the starting point to not be ignored. Part of this must be from early participation in Steemit etc.
Then I discovered bid bots. Whammo blammo, instant rep increase from 35 to 60 in a week, at a cost similar to my initial 1000 SP. It still doesn't mean anyone actually reads my posts, let alone uses my (free) stuff in their lives, but it seems like getting to square one.
And it's not even really a sunk cost, money down the drain. The money just moves from one asset class (fiat, ethereum etc) to another (SP essentially), and it can be slowly recovered through powering down again if desired.
Often the people who use them don't engage in the community and expect eyes on. It can help with eyes but if one doesn't concurrently put the time in, it means that there is very little value other than the financial ROI. I think that as communities become a feature in the next 6 months there is much more chance to develop niche content areas.
For most, it is going to pay off very well in the next run. There is an opportunity cost of other investments but that also has to be placed alongside ability to earn on top here relatively risk free too.