My card allowed me to diagnose three and fix two 286/386/486 systems in one month. Yes, worth it.

Buy whatever you can find cheapest from China, they are mostly the same card.

About the usefulness, yes and no. If it's a clear fault at one of the POST stages (i.e. memory fault) then it's possible to narrow down the problem. However, if the problem is the mainboard itself, things get more complicated as the POST might hang at different stages and give you different error code each time. Then it becomes somewhat difficult to determine where the problem lies. Tt could be the voltage, voltage regulator, tantalum caps etc. and therefore you need to start hunting the actual cause with a multimeter.

This.

Also, knowing which bios your motherboard has and sometimes which model your board is can help. I saved a socket 775 motherboard that way by looking for the error code. I also saved a 386 DX motherboard, a 286, and some 486s that way.

If a motherboard doesn't post, but outputs some codes this is already a good sign because that means there's still some life in the board.
However no code doesn't mean that the board is toasted : once I had a socket 7 mobo that didn't do anything ... Ended up finding that the bios chip was toasted so I flashed a new one and it worked !

Buy one that has ISA and PCI connectors at the same time.