We use an iPad in our house (we have two in fact). We also have the blackberry tablet and the kindle but iPad works best with my toddler.
My son uses it for a very limited time and with supervision. I was not a big fan of tablets for children but for us, the good outweighs the bad. Case in point: I spent a lot of money buying a good quality Montessori World Puzzle Map. My son mastered it in minutes and he wanted similar puzzles given his interest in the world map. I bought some apps by
http://www.mobilemontessori.org/ (I really like the apps for the most part). While he memorized most of the world map (from a physical map), thanks to those lovely apps, he is able "solve" various continent puzzle maps for a fraction of the price and less clutter at home. In the hardcore Montessori way, it isn't the same but this adaptation works for me.
My only suggestion with the iPad is to buy the largest screen and most memory one can afford. I use my iPad a lot more than I planned.
Great to know about the Montessori Apps! We're saving up for an iPad for speech apps and school use and I'm glad to hear some of this info. I figure the good will outweigh the bad, ,as someone else said, if we are mindful about it.