We have an iPad 2 and then an off-brand Android tablet (that only works when plugged into a power source, lol).
Basically, my iPad and my iPhone can do pretty much the exact same things (save making phone calls), except it is much nicer watching movies and surfing websites on the iPad. Certain games, if you like such things, also offer much smoother play on the iPad. Likewise, certain apps just feel better on the iPad. Typing is much easier, so anything that involves writing works better on a tablet.
I use 3 G all the time on the phone, but hardly ever on the iPad. Basically it is too big for me to take it out when the subway is at maximum capacity on my morning and afternoon commute, so I don't usually even bother bringing it with me. The iPad comes in really handy, however, on long trips -- train, plane (where they won't let you play on your phone, but they have no problem with tablets, which they seem to consider to be computers), car, etc. You just need to make sure you have the media you'll need loaded up in advance -- and 3G isn't fast enough to stream movies or anything like that anyhow.
They are also, no kidding, a parent's best friend. Our iPad is like the nanny I could never afford, lol. It can keep the kids entertained for hours with wholesome educational apps. My 5 year old has learned about 100 Chinese characters with an iPad learning program. The cell phone screen is too small for kids to really manipulate effectively (they can play fruit ninja and angry birds, but nothing complex) but the iPad is just the right size.
It can also be a cool teaching tool. I know teachers who hook the iPad up to the screen in class and use interactive learning apps. I haven't done it so far because I don't have the adapter, but I might get one for next semester. Don't think you can do that with a phone.