I have used myIntervals for years. I LOVE it. Tip: Use one project for each client, then use tasks as "projects" for that client. Saves you a ton of money. They also have milestones (which I use more as a to-do list.)
I really love that fact that I can add as many subs as I want as users since myIntervals charges per project not per user. Also clients are allowed to add and comment on tasks (many other PM's do not allow this function
They have GREAT reporting and invoicing features.
If anyone has any questions on using myIntervals feel free to send me a PM.