The OWI arm takes a couple hours to assemble. A bunch of injection molded parts you have to pop out and snap together, along with a few screws in some places. The motors are just plain DC motors, no feedback or anything like that. My kids love playing with it.
I created a very simple Node API for working with the OWI Robot Arm + USB kit, you can get it from npm or from github.
https://www.npmjs.com/package/owi-robot-arm
https://github.com/bigbang/owi-robot-arm
I also had some fun with a project this summer turning LEGO Mindstorms EV3 kits into connected devices. They can send realtime telemetry to your mobile device, browser or other robots. We have done a couple pilots with them in classrooms as well as taking them to places like Maker Faire and Mozilla Hive events.
The Mirobots look really interesting, I definitely will check those out. I can also confirm the Snap Circuits kits are tons of fun, and my kids love playing with them as well.