Useful

Fits when convenient availability or easy use deserves informal praise

Careful For a major tool or formal evaluation, handy sounds too slight.

Fits when assistance to a person is more important than the object's function

Careful Helpful can describe kind intent even when the result has little practical value.

Fits when the benefit justifies the time or effort invested

Careful Worthwhile evaluates the overall tradeoff, not direct usefulness for a task.

Fits when evidence shows the thing produces the intended result

Careful Effective is a stronger causal claim than merely useful.

Fits when something performs its basic intended operation

Careful Basic operation need not make the result useful to this reader.

Fits when suitability to real conditions matters

Careful Practical contrasts with theoretical or ornate, while useful simply means it serves a purpose.

Fits when importance or benefit is substantial

Careful Valuable is stronger and may refer to money, rarity, or significance rather than utility.

Fits when adequate performance, not distinction, is the honest claim

Careful Serviceable implies mediocrity and would undercut enthusiastic praise.