The e-CFR is recommended when a quick answer is necessary, and only when that quick answer can be followed up with additional research in the other resources.
Finally, one additional area where the e-CFR is lacking: pending regulations.
The Electronic Code of Federal Regulations (e-CFR) at www.gpoaccess.gov/ecfr/ is a project that, while not the authorized version of the code, is a prototype for the near future.