Digital thermostats for underfloor heating: useful? Is the extra cost justified?

... Or do you really use them and occasionally adjust the temperatures? For example, 18 in the bedroom today, then 20 tomorrow, and 16 next week?

Not in the bedroom or bathroom, but in the office/gym/guest room, so a multifunctional room. For sports, the temperature is turned down. Works quite well with parquet :)
And then there are personal situations where you prefer it warmer, for example on vacation or during weather changes. In those cases, you set it one unit higher for a few weeks.