No reason not to if you are in a position to switch to them as a carrier. Just check their coverage map.
The three compatible phones (Nexus 6P, 5X, and existing Nexus 6) are great phones specs-wise (read reviews before picking one), and usable on any network...they were not specifically built for Fi. I actually just got a rid of a Nexus 6 to upgrade to the new HTC 10 because I'm a bit of a new phone junkie, but the 6 was great. Easily the best camera I've ever had on a phone, and the stock android photo editing capabilities for touch-ups are outstanding.
If you are in their map and aren't on another contract, I would go for it. Monthly plan pricing looks reasonable. The discount they are offering on new 6P's and 5X's is great, too.
For a domestic user this probably doesn't make much sense from a monthly bill perspective. For a user that travels internationally on a regular basis, it is an incredible value and simplifies international connectivity in a revolutionary way. Personally, I hope that the traditional carriers lose enough customers to Fi that they feel a need to offer a competitive alternative. I'd love to have a similar option on my corporate-tied AT&T plan.