I like it a lot, it's how I tuned my house enegy usage.
You're right though, there are small donut CT's on each breaker, and you'll do a one-time cutover for these by doing exactly what you said (breaker-off, undo wire, put in CT, reconnect, breaker-on).
For the Sub-panel, the main line uses Split CT's so that's easier.
For the Main-panel, the incoming lines are on large/flat square bars, so even the normal (round) Split CT's wouldn't work. I monitor all the sub-circuits there so I ended up leaving out the mains (and computing it later)
All the CT wires themselves come out the bottom of the Breaker box, and then into the GEM.
Configuration-wise, the (newer) GEM is a lot easier to setup than the ECM-1240 is. The latter requires a Windows box to run the config tool whereas the GEM has a Web Interface.
I have a Rainforest Eagle-200 in a box somewhere. I used to read the SmartMeter directly to compare results but PGE changed the meter out at some point and knocked this device off the net. One of these will let you see/detect variations in base-load, so you could combine it with a few per-circuit things to get an overall picture.
The components of my system are:
