Do you find them useful?
Blaise
Yes and no...
For shrimp, when I'm dealing with adding specialized mineral salts to RO water after a water change - yes.
For fish? Not so much. I can use them to rule out that some sort of high-hardness is the reason behind something going on in the tank, sure.
If you do most of your changes with tap water, and you are using rain or RO from a functioning RO unit (which should have a TDS meter built in) to do a partial water change on a tank and mimic rain, then your rain water or RO water is lower TDS than the tap, unless you're on a well possibly, and it's a drop in TDS. I don't think it's usually the exact level - it's the drop. In some cichlids I've had issues with egg "shells" being too hard in hard water, so the fry expend too much energy in hatching (if they hatch at all) and die within the first day or so.
As an aside, I've discovered some interesting stuff using my TDS meter - for example, heavily planted tanks soften over time if you don't keep up with water changes. This does not mean the water is getting "cleaner" it just means the plants take up a lot of those minerals. That means constantly low TDS isn't always a great thing for fish (does depend on the fish, though). Rooibos tea can raise TDS a little (high mineral content). If your fish are used to part RO or part rain, you can trick them into a "dry season" by using tap for a month or so and then doing a big change with some cool, low-TDS water to simulate a big rain.