What do you guys think about them? I for one am happy they added something like this to the game. I have over hundred of them. They are fun to use and fun to test out settlement defenses.

They're fun, I won't deny that, but they only relay the weakest Gen I synths, and because of that the item is quite useless in late game. I wish they patched it so it spawns an appropriate Synth variant according to your level. Like Synth Patroller for 25-40, Synth Trooper for 40-50 etc.

can you make these?

You buy them in the Institute after you completed the main quest. You can buy 30 at a time then you can wait two days then buy them again. 30 cost around 2k. It's worth it though since you can spawn an unlimited amount of them which is surprising.