It is easy to use given one reality: Robots live in the real world. What I mean when I tell that to the students is that each robot produced is slightly different. (They are cheap.) It is also the case that sometimes the robots do unexpected things one time and never repeat it again. It can also be a challenge to debug without putting some thought into it.
Most of the big brands do come with projects of some sort. Given that most of the robots are very similar you can even use projects from other brands or find projects on the web. You just need to figure out how to incorporate them into your curriculum. I don’t usually give solutions to my projects so that students cannot just cut and paste, but most do give out the code.
There are several versions of the DFRobot Maqueen robots at different prices. Depending on how flexible you want the robot to be the higher the price usually. I use the Maqueen Plus V2 which is about $50. The Maqueen Plus V3 is new and costs $70. There is also a Maqueen Lite at $32. I am right now evaluating the Elecfreaks Cutebot Pro which is also $50 but has more functionality compared to the Maqueen Plus V2.
So cost effective? With a micro:bit and rechargeable batteries I paid about $80 per robot. We have had them for 2 school years so far with one broken bot that cost about $30 to repair. I would say it is cost effective.
One thing that DFRobot has done to make me want to switch products is they produced a bunch of Maqueen Plus V2 robots that didn’t work properly. They obviously didn’t bother to test them after making some manufacturing changes. I have 9 of them with this bug. When I opened a bug report it was ignored by DFRobot for over a year. When they finally replied they told me they were sorry but I would need to fix it myself. I can do that, but not everyone can.
To program a robot you plug it in via USB cable. You use a web based development language. MakeCode or microPython are available easily, other more esoteric languages with some effort. My school gives all the kids Chromebooks and it works great. Laptop, desktop, tablet, and phone are all supposed to work, though I would not recommend a phone. The micro:bit themselves can connect to a tablet with Bluetooth so you don’t need to plug it in. I have not tried that yet because I don’t need to do that.
In the classroom I use the robots to offer an alternative to a MakeCode Arcade project. Some of the kids prefer to do that. Oddly, the girls seem more attracted to the robots than the boys. I was not expecting that.