"You'll never sleep/ have a minute to yourself/ get dressed before lunchtime/ get your figure back again"
Load of rubbish. I have never been so well slept since having kids. They woke a bit in the early months to feed, but they are excellent sleepers and I dont have to get up at 6am to commute to London and go to work, so consequently I get about 9 hours a night which is far more than I got working in London and socialising.
As for having a minute to yourself - true in a sense but if you have family help or friends you can arrange favours, and if you have a reasonable disposable income you can pay someone to have them once a week to give you a "me" day if you need it. Hell, they are at school before you know it.
Getting your figure back - of course you will, you might have to diet a bit. You might not look the same naked again but it wont be a disaster either. There is no reason why you can't look as good in clothes as you did before as long as you are willing to do a bit of diet and exercise if you have stubborn fat stores.
"You have NO idea what you're letting yourself in for"
I think you have a fair idea - presumably this person is not the only one in the world with children and therefore you have observed children and families. It might be different in reality but to say you have no idea is patronising and stupid.
"Oh well, you better get used to it, soon your house is going to be a shithole anyway"
Only if you want it to be/let it. There will be some fairly repetitive thankless cleaning tasks particularly when your baby is about 9/10 months old and chucks their food about 3 times a day and gets Weetabix on the wall and that malarky but it will only be a shithole if you choose not to wipe it off. My son had reflux yet my carpet is stain free and fresh smelling. Yeah, I had to buy a Vax to achieve that but I rather like the thing! 
There might be toys around but bad as a heap of fluro plastic makes a room look, it takes about 5 minutes to whip them away in a basket and get on with your evening.
I know you didn't need reassurance as such but just a rant about the cheek of people, and I wholeheartedly agree, but also hope I've put your mind at ease that their experience may be vastly different to yours.
I always wonder why people go on about these things specifically when the really challenging stuff are more to do with the deep bond and love you feel...the almost panicy sick feeling you get when you leave them for the first time, the wrench in your gut when you are standing in the preschool foyer and another child says to their father, "I dont like *" (your child), the total feeling of complete nausea when you watch your child get up and sing in church or the wishing beyond all reason that you could have taken that bump on the head for them. Its those things that change you forever, not carpet, Sunday lie-ins or being able to wee on your own. Most those logistical things can be achieved if they are important enough for you - you are the child's mother, not their slave, and you are in charge! The things that are important to you, you will acheive somehow, eventually.