It depends what the object is that is being useful ... a tool is useful to you... an activity like exercise is useful for you

It depends what the object is that is being useful ... a tool is useful to you... an activity like exercise is useful for you

This's nice! Thank you!

Either works :) Perhaps "useful to" would be a good first choice :) https://books.google.com/ngrams/graph?content=useful+to+me%2Cuseful+for+me&corpus=26&year_end=2019&year_start=1800&smoothing=3&direct_url=t1%3B%2Cuseful%20to%20me%3B%2Cc0%3B.t1%3B%2Cuseful%20for%20me%3B%2Cc0#t1%3B%2Cuseful%20to%20me%3B%2Cc0%3B.t1%3B%2Cuseful%20for%20me%3B%2Cc0