Worth money, but most I've seen are overpriced. And most testers only include 1 of each switch type. Pressing 1 switch over and over does not give you a good feel of what a full board of that switch feels like.
I agree with this. 4 keys and a bunch of keycaps by themselves are really quite cheap but most places sell them overpriced.
I bought a switch tester before I got my board and I'm really happy I did. After a week or so of playing with it and letting some friends try and all that, I pulled the trigger on the red switch. Again, this was based on how 1 switch felt by itself and not a cluster to press together.
If you're new to this hobby and am interested in going deeper, a switch tester would be useful in letting you feel the difference between each keys. For example, if you only have a keyboard with reds, when someone talks about how small the tactile bump is on a brown switch and how people should really try clears instead of browns, you'd have no idea how tactile a brown switch really is etc, even if it's just an individual switch and not a whole board