I've purchased several arrangements for my pop a cappella group from ChorusOnline. It's always been a super-easy process. The materials are thorough, music and practice tracks included, and arrive quickly to my inbox.
Strengths
ChorusOnline has high-quality arrangers in contemporary a cappella, versus much of the watered-down pieces you get from typical publishing houses. Best of all--arrangements are licensed! For what it cost to arrange and licensed one song myself, I can purchase SIX from ChorusOnline! Love that.
Improvements or suggestions
Continue to offer great arrangements!