Just got my hands on the Raspberry Pi Model A+.
Here is a photo comparing it my old Model B. (Still working on a robot project.)
The A+ is smaller, cheaper and lighter than the B and B+ models. You lose extra USB ports, ethernet, and you have less RAM (256 MB vs. 512 MB).
Setup was really easy, as I had cloned my previous Pi’s SD card using the procedure described here. Then, as described in my note about setting up the pi the first time, I just changed the IP address and I was good to go! (Since it has only one USB Port, I first used it for my USB wireless keyboard and then plugged in a USB WiFi adapter after I was done with setup.)