Let's start by saying that slavery is of course a terrible thing, one of the worst crimes imaginable, and that "slavery" rarely implies good treatment, anything noble or defensible. Nowadays, even god doesn't escape judgement from our enlightened views with passages about slavery in the bible usually glossed over because they are shameful.
But not all slavery was equal.
Because of the media, movie industry, racial tensions in the US regularly shown on the news and our taught history, I think most people's knowledge and idea of slavery is that of the North Atlantic slave trade where white people took Africans to work in cotton fields. This idea is probably also re-enforced because the largest group of descendents of slaves we see today are African Americans (usually in the US or places they subsequently migrated to).
But it's incomplete.
It's only when you look into it more that you discover that there was much more to the slave trade than that, otherwise it would have been just called "the slave trade" and not "the North Atlantic slave trade".
Some 12.5 million slaves were taken from Africa to the US with just under 11 million surviving the trip so it obviously took a terrible toll immediately, even before any maltreatment once they landed in the Americas where conditions and treatment were truly awful. Slaves were an investment and needed to be kept alive though.
But they were only 5-10% of the total number of slaves taken from Africa. It's estimated that up-to 120 million may have been taken over the centuries. So where did they go and where are their descendents? Where are the African-[somethingelse] people?
The other main destinations for African slaves was South America where they were viewed as expendable labour and rarely survived for long periods but for the majority it was the Middle East where the future was even more bleak.
The survival rate for their trip was less than 40% (they were mutilated to prevent breeding and few survived) with a subsequently very short life expectancy in barbaric conditions. They were cheap, replaceable and no investment. There are few who can trace their lineage today because nearly all of them died at the hands of their non-white, islamic masters. There was no abolitionist movement in the Islamic world. It actually persisted in many countries and was only finally stopped in some as late as the 1970's (!)
So, although it's hard to imagine, if you were an african slave being loaded into a boat you should have prayed to god that you were going to the US and not someplace else.
I'm sure that is little comfort to those who died or were subject to cruelty and oppression. None of those who survived could probably imagine their offspring would one day enjoy freedom and equality in a country that others would dream of getting to. Slavery will always be a stain on America's past and any other country that played a part in it before fighting for it's abolition. But thankfully, millions are alive today because their ancestors endured and survived.
Also pertinent: http://www.globalslaveryindex.org/