Try the app "Text Now", which can be installed in both iPhone and iPad, even at the same time. You get assigned a phone number, and can use the app to send regular SMS to others. They do not need to install any apps, just use the regular SMS app to communicate with you.
It is free, with ads. Oh, you can also make free phone calls, and even free long distance call in UK, US and Canada.
For me, it's great for Craigslist use when I don't want to give out regular my cell phone number. It's kind of like a burner phone.