I'd say to just start off with a couple DC motors and a motor shield to controll them You can do most of what the other platforms offer on your own with minimal effort. You can print a fancy round frame for it if you really want one.
I like the look of the two-wheeled Turtle from DFRobot, along with their 2A dual motor controller.
The MAKE Rovera, which got one negative review, is actually a bigger, more expensive kit that appears to include the Turtle along with some other stuff, so I wouldn't let that one negative review cloud the DFRobot offering.
@Max: If you are ready to build your own robot from the ground up, let me offer my own approach.
I'm a LEGO fan since I was little child, and when I need to prototype something similar to a robot, I use to attach some bricks to the MCU boards until I get a nice Frankenstein-like thingy.
The last time I used LEGO plus electronics, I was trying to build a helicopter form factor drone: I bought a simple and big RC helicopter, a wifi enabled webcam and some LEGO Technics boxes to update my collection -- of course, the helicopter was destroyed in the first test flight ;-)
NOTE: In addition to LEGO Technics, now there are available kits for robotics such Mindstorm and NXT.
@willb6: I like the look of the two-wheeled Turtle from DFRobot, along with their 2A dual motor controller.
Thanks for the feedback, but I've sort of gone a different way -- I found an interesting base with three wheels (those strange wheels that have sub wheels on the main wheels) -- then I found a motor controller from AdaFruit that control four motors -- I'll be writing all of this up in a future blog.