I'm currently evaluating intervals, and I love the views it gives me into where the project is at and what work has been done. We currently have a few hundred open projects, with lots of time already tracked to them.
What's the best way to add those projects and get the existing block of time in place?
Or should I just use the legacy system for all current projects, and start intervals for new projects going forward?
Unfortunately there is not a way to import old projects and time data at this time. You could potentially manually add them but depending on how many details are contained it could be a bit tedious. If the manual route turns out to be a possibility you may want to enter the time for each project in one bulk entry as opposed to spreading it across weekly timesheets. Adding the total time will at least give you the total hours used for the project but not necessarily the exact day the hours were tracked. If you happen to be a web developer the API is currently in beta and could potentially be utilized to import data. Not great options but hopefully it helps navigate the options a bit.