Well in my experience the babasling and the lifft sling were most useful up to about 7-9 months, when ds became too heavy to carry on my hip for an extended period of time, however the lifft was really handy to have (it's only one piece of material which is quite stretchy so it's not too baggy when there's no baby in it, so I used to just wear it over my clothes in case I needed to carry him). Dh loved it as it was really simple to use.
The mei tai can be used for newborns I believe, but I didn't get mine until ds was a bit older. Carrying an older baby on your back is so much easier than on the hip and ds liked being able to get in and out as he pleased (more so with the oh-snap as it has buckles rather than long ties).
I hated taking the pram on public transport or into the nearest cities so it was so much more convenient to have those. My mei tai was a custom design and cost me a fortune (plus bloody customs charges!) but I love it and will treasure it always!
The kari-me was bought too late to use for the intended purpose (I got it really really cheap
) so I'll be testing it on this baby! The babasling had too many straps and buckles to be really comfortable which is why I want the ring sling too. I think between a ring sling, kari-me and mei tai I'll have plenty of choice for the newborn stage!
So after all that I would say I definitely prefer simple (so one piece of material) over lots of adjustable straps and buckles etc.
However, dh is the opposite and likes the straps and buckles as he just sets it to his size and doesn't have to think about it or remember how to do the many different ways to tie the slings.