Personally I got a decent amount of ante natal care/information but my mistake was to read the guidance that says the first year is the hardest, so pretty much at their first birthday I was congratulating myself on getting there and never suspecting that it would get more challenging. I think there was a comment in one book that once you've got through the first year it should all become easier as the children become more independent. This couldn't be more wrong for me - I found the early months relatively easy, in that it felt that being organised was half the job (crowd control, essentially). At a year old they're hardly 'independent' anyway! If anything, more likely to get injured by falling/pulling things down on themselves.
Once they became mobile and their requirements went beyond sitting in a buggy or on a playmat with a toy to look at, I found it so much harder than before. It was always tricky to do activities that mums with one probably take for granted (playgroups, swimming, massage etc were either exhausting or ruled out altogether) but even things like meeting friends for a coffee or going to a playground became so hard to do - not enough pairs of hands, children going in different directions, even simple stuff like getting us all into coats and shoes to leave the house...there were times when I felt I couldn't face all the faff and probably became quite socially isolated.
Returning to work and facing the issues of them catching bugs at nursery was immensely hard too. I was unbelievably stressed about it all, trying to do my job to previous high standards while getting calls almost every other day (seriously, in the early weeks) to come and collect one or other. The fact that illnesses are usually staggered by a couple of days means there's quite an impact on ability to work if they can't go to nursery.
I think it is probably underplayed that you will need help to manage twins. I am beginning to see now (2.9 years in) that I am going to have to find some solution - I have no willing grandparents or other family on hand, so it needs to be paid-for help.
I'm having a bit of a hard day today as you might be able to tell from the tone, but for me there has been a massive hit to quality of marital relationship (we separated for a while earlier in the year), my physical strength (lugging two babies around is hard and I have quite bad posture now), my mental resilience and my immune response. I seem to catch all manner of bugs now and have had conjunctivitis for 3 months...
I imagine my husband would say it has had a huge impact on our relationship too, and has brought to the fore certain shortcomings in our parents that we hadn't noticed before, and that we are constantly short of time.
Funnily enough I don't sweat too much about the point Gluezilla made about how much time you have for them individually. One area where I give myself a break, perhaps - I know that they are happy, thriving and seem to have a lovely bond with each other, and will never have known any different anyway.