It's great. Expensive, but great. You need to remember that it isn't just the cost of the bigger house and car, but the constant cost, which gets bigger the older they get. Mine are 8-15 and some days I feel I am hemorrhaging money!
If you stay in a hotel, most hotels require you to take 2 rooms. If they're going to go to Uni, that's 4 sets of University educations you've got to be ready to stump up for (and in our case, there will be a year or two, when 3 of them will be there at the same time). Etc etc.
I don't have much time for the whole "they don't get all your attention" gumph. I was one of 4 and that never bothered me. It doesn't take much for a parent to show their child how much they care on a daily basis. And, when you can't do it, they have each other. As long as they're getting on that day, at least. 
It's not for everybody, I am happy to admit, but I don't regret it for a moment.