I only joined this site a few days before you so haven't the experience of all the others on this site however my son has been on replacement meal drinks for about 7 weeks now(just coming to the end of re-introducing 'solid' food into his diet again.
The drinks that he has been on are made up of partially digested foods(when I was first told about them all I could think about was how birds regurgitated food for their young-yuk) and I believe their use is two-fold, firstly it gives the digestive system chance to rest(and hopefully heal) and secondly because most of the hard work needed for digestion has already been done it allows the body to absorb and use the nutrients in the drink. Each one of my son's drinks came in a 200ml bottle and provided him with 300 calories (he was on 8 per day) I understand that some people(esp children) find them un-palatable but they come in a whole range of flavours and if chilled do not taste that bad.
Once food has been fully re-introduced he is to continue on one of them per day(for extra nutrients) and another type of 'shake' is added -it comes in a sachet and needs to be mixed up with 250ml of full fat milk (still wonder about that though with all I have been reading regarding cows milk) and that wil give him a further 900 calories a day (trying to beef him up a bit!!) The bonus with these is that there is a neutral flavour which means it can be used (tastes like milk) on cereals in recipes etc.
The make he is on is Fresubin, here in the UK he has it prescribed for him, (I do know that it can be purchased online though) but there are quite a few other makes available too.
As he was relying on them for total nutrition we aimed for him to have 2 am, 2 mid-day, 2 at tea and 2 in the evening(although that did not always run to plan!)