Some months back, we did an "Introduction to Arduino" then later a complete class teaching our members how to build a robot from scratch. Most of the parts were ordered from Pololu.com, as they offered us a great discount on shipping and then an additional 10%, making it possible for us to create a nearly $60 kit (with shipping) for under $40 per person. The only parts we didn't provide were the Arduino board and batteries (as most people have them already).
The Tamiya gearbox assemblies we ordered were of high quality plastic, metal and even came with lithium grease, but during the class we faced many hardships. You can find great reviews online (these really are good gearbox assemblies at a decent price), but I thought I'd give my 2 cents about why we probably will not be using this model in the future for large classes and give a list of solutions in case you run into the same problems as we did.
1.) Assembly took roughly 90 minutes. It was a great exercise in patience and eye/hand coordination, but I don't think it's something I need to do more than once in a lifetime.
2.) Working with 40 people in a presentation room means when you drop one tiny screw or gear, you're out another 10 minutes crawling around on the floor searching for it.
3.) There are NO EXTRA PARTS. If you lose something, you do not have a working gearbox.
4.) A few people complained that the boxes they received had been opened already (I was the only one with access and didn't tamper with them) and there would be a small grommet or screw missing which impeded the functionality of their Robot.
5.) The bottom of the box is left open, which in most cases is fine, but considering our robots were running around in non-sterile environments, any dust, hair, carpet fibers or dirt from the ground would stick to the grease and create wear on the teeth of the gears over time. We were able to remedy this mostly by applying a few short strips of masking tape to cover the opening. Just make sure the tape doesn't touch the gears.
Now, in the future, we are looking into going with a completely contained gearbox assembly that comes in two separate parts so that they can be spread as far apart or butted up against once another if needed. I've already spoken to someone about getting a few to test with and will post an update with my findings.