Have been using Directi with both APi and Store Front(for resellers), most of the probs where mine though.
Have got a great cart with whoiscart.net Part of the deal is you get dropped straight onto the 6.75 level at directi. The cart handles directi,enom,godaddy soon,centralnic,opensrs,bulk register. The integration is usually your config file.
You can run as many registers throught the cart as you want, directi for .com and enom for.us . The clients are registered with you. It was easily the best $35 I have spent for awhile.