These services tend to be reliable and I believe it's the best way to convert SBD into STEEM POWER.

I only use it on posts where I have the rewards set at power up 100%. Some things that might be useful to know to maximise the return. Make sure to have a look at the @minnowbooster voting power. (it was at 50%+ when I used their service on this post and it gave a 8$+ upvote for a 5 SBD price.Also make sure you use the service within the first 15 minutes so the curation rewards go to you. after 30 minutes will lose 25% of the vote towards curation rewards.

So 5 SBD is good for around 8$+ of Steem power after 7 days and you get some extra post attention as a bonus. If anyone knows where to get a better deal please let me know !