I have iPad 1 and 2, iPhone 4S and iPod touch 4. My only regret is upgrading iPad 1 to iOS 5, not in buying any of the devices.
I use my devices differently. I like touch because I have all my audiobooks, books and music in a small package and can use it without worrying about running my phone out of juice while on the go. I often carry only iPhone and touch, because iPad is too big. I use iPad more for travel and home, rather than day to day on the go. I find the iPod touch's screen too cramped for typing and navigating, but love it for audiobooks, music and books (hook up to car stereo while driving, or read one-handed while on subway, for instance).
I also keep my contacts and email on iPhone, not touch, because I can worry less about it being stolen while reading on the subway, for instance.