Barbie - yes I have had a baby bjorn.
After using Tomy carriers, I found a Bjorn a lot better - easier to get on, fool-proof and carries baby a bit higher. It kind of introduced me to carriers, so I do respect them in that sense. There is nothing particularly wrong with them, but I've since found much better carriers that hold the baby higher up, let them sit in a comfortable position - with their knees higher than their hips (rather than them dangling by their crotch), are useable for a far longer length of time, are more comfortable and supportive for the wearer, and can be used on the back as well as the front. There is also an issue with spinal compression with structured carriers as the fabric may be too rigid to mould to, and support the spine.
In saying all that, though, since they aren't the most supportive, you probably wouldn't wear it long enough for the hip placement and spinal compression issues to be a problem. There hasn't really been enough research done on these things anyway, but interestingly, Baby Bjorn have now launched a carrier which mimicks the positioning of the other carriers here - note the more seated position, but the baby is still quite low down.
Anyway - sorry for rambling - in a nutshell, they are good because they are sooooo easy to use, so are fab for introducing someone to carrying their baby in a carrier. There are, IMO, far better carriers out there, but if someone has given you one for free, it could be useful at the start, and if you enjoy carrying your baby, you could progress to another carrier after a while. They sell well on ebay too.
Hope that wasn't too rambly and confusing 