Not sure if this is the same thing you are talking about, but we had a co-sleeper cot for our baby last year - It certainly replaced a cradle or moses basket, but I can't imagine how it would replace a travel cot or playpen, so maybe you are referring to something else?
Ours is a Cosatto cot, with a drop side that drops all the way down and stores in under the cot (handy for storing things like baby clothes, nappies etc on in case they are needed at night time.) See here:
http://www.boots.com/webapp/wcs/sto...toreId=10052&catalogId=11051&productId=120225
The cot pulls in right beside our bed and her mattress was exactly at the same height as our mattress, which meant there was no gap or barrier between me and her - if she stirred at night and threatened to wake up, I could just slide my head in beside hers for a couple of mins and my breathing right beside her would normally settle her.
The big advantage though was for the night time feeds - I didn't even have to sit up at night to feed her - just reached over with one hand, slid her across from her bed into our bed, quick feed, and then slid her back into her own space again. Easy peasy!
Now that my daughter is older, it functions as just a normal cot in her own room.