Usefully rhymes with:
blissfully, forcefully, gracefully, peacefully, purposefully, successfully, unsuccessfully, anencephaly, artfully, blissfully, carefully, cheerfully, delightfully, dreadfully, dutifully, faithfully, fitfully, forcefully, frightfully, gainfully, gleefully, gracefully, gratefully, harmfully, helpfully, hopefully, joyfully, lawfully, manfully, masterfully, meaningfully, mercifully, mindfully, painfully, peacefully, pitifully, playfully, purposefully, regretfully, respectfully, rightfully, ruefully, skillfully, sniffily, successfully, tactfully, tastefully, tearfully, thankfully, thoughtfully, truthfully, unlawfully, unsuccessfully, wilfully, willfully, wishfully, wistfully, woefully, wonderfully, wrongfully