The hard part was removing the old surrounds, cheap spring-loaded crap that the builder installed. Once that was done, installing the lights themselves was very simple, except for a few of the fixtures over bathtubs, where getting the ladder situated properly was more challenging.

Note that some of the newer Cree bulbs come not with a traditional Edison screw but instead with a GU24. They include a GU24 base with bare leads and some wire splices alongside the bulb. You'll want a basic wire cutter and pliers to do the modification.