Well, I'm just a year in with baby #2. So she's 1, and her older sister is 4.5.
So far, the only downside I can see is that they're not old enough to play together, and they may not have much on-par play lives given that one will almost always be more developmentally advanced.
But I have to say, so far, with two girls, it's been great. No jealousy issues, so far, either.
The older is in FT preschool, and the younger is with a nanny. Ma and Pa knock off by 5pm, so we have time together as a family almost every night.