The experiment results weren't all that impressive.

Literal

Experiment [possessive-の] results [topic-は] not-quite impressive-not-were.

今ひとつ 'not quite, somewhat lacking' softens the criticism. ぱっとしなかった 'wasn't impressive, didn't stand out' — ぱっと is a 擬態語 for something flashy or eye-catching, and しない negates it. Together they politely express disappointment.