I'm playing a Time/Sonic, up to the low 40s (forget exactly), and I'm pretty impressed overall - Time offers fantastic buffs and debuffs plus a solid amount of healing, while Sonic is both a good single target set with some control for when you're soloing, and a huge force multiplier against tough targets while on a team. There is a small anti-synergy in that with Time you really want to be in the middle of the enemies so Time's Juncture can be at its best, while Sonic's AoE powers (barring the nuke) are cones, but it's not that bad, and even a Howl fired from inside a group will usually hit a good number of targets.
In terms of powers, I took all of Time, skipped Shockwave, Amplify and Screech from Sonic, then Hover/Fly, Hasten, Maneuvers, and Power Build Up/Temp. Invulnerability, with Boxing/Tough/Weave planned for the last few slots.