We have a Baby Bjorn, an Ergo, a Hug a Bub and a Goldbug Sling and Hip Carrier - the Bjorn and Goldbug were presents so luckily I didn't have to pay for them!
Out of them, we use probably the Ergo the most - she sleeps in it easier, and it's great for my shoulders/back. I also use the hug a bub at home - it's extremely comfy - if you tie it correctly!
The Bjorn has become my DH's carrier, it's great when we are out and about and it's not her nap time - she can look outwards. I think he finds it comfortable enough, but I find it digs in at my shoulders. Also, as PPs have pointed out, it isn't great for bubs hips etc cause they sort of hang there as opposed to sitting in it, like they do the ergo.
I bought my Ergo second hand - I wasn't going to get it because I had heard the hype but thought that it was probably overrated, but then I saw it cheap and thought 'why not' - now I'm glad I bought it!