My 3yo plays on my iPad a lot. She has more apps than I do! Lol. She has a speech delay, and I have a few apps that address that. I also have apps designed to work on fine motor and cognitive skills, preschool math, letter recognition, etc. She has one app on it that is purely "for fun" (no educational purpose...) but even that has helped with her speech development. There are LOTS of games that are educational, so you can focus on installing those if you prefer.
I only paid for one app, because she really liked the free content, so I upgraded to the full version. The rest are all free apps or the lite (preview) version.
I limit her time on it when I can, but it is GREAT for trips, or when I need her quiet and preoccupied while I work on my homework. :)
If they love to read, there are apps to turn tablets (and iPads) into e-readers, and e-books are often cheaper and easier to access than paper... And many libraries have started loaning them.
Personally, I think it is a good idea to introduce kids to technology, and how to appropriately use it, when they are young. It is such a part of every day life, that they will need to understand it anyway. It's much easier for them to grow up with it, than to teach them from scratch when they get older.
So long as you put limits on their use, I don't see a problem with me having one. You can put locks on it to limit what they can access, so that's not a worry. I'm not sure if I would buy one "for" them though. Maybe get one, for the whole family to share.