I've tried one before. It is comfortable enough for a no-frills carrier and is a good size for a larger baby. It is only marketed as being for front carries, but I didn't see a structural reason to not use it for back carriers if you cross the straps across your chest in an X (which isn't always comfortable or attractive when it comes to breasts). It doesn't have much in the way of padding, so--for me, as an extensive babywearer--I would consider it as a car carrier (something you stash in your car for when you forget to bring something else).
I've also tried one. I agree it's not really ideal for back carries at all (thus it's not marketed for them). It also isn't good for petite wearers. The straps were almost too long for me with a toddler and I'm not petite! It is a pretty comfy front carry though and I'd recommend it if you are looking for a front carrier only for a larger infant or toddler.
Re: Moby to go
More Green For Less Green
Breastfeeding Counselor with Breastfeeding USA
Babywearing Guide ** Newborn Carriers
Cloth Diaper Guide
Safe Bed Sharing Info