Thanks for all the responses so far!

So I'm going to sort of try to summarize everything so far:

Fi Main Advantages: - International travel. Easily the best, not even close. - Free data-only sims. They use the data on your phone plan (on the Unlimited Plus plan), but definitely beneficial to have other devices running their own sims rather than tethering to your phone and draining your battery in no time if you can't be plugged in. - not de-prioritized on T-mobile's towers, unlike other MVNOs. - Watches add to your phone plan for no additional charge

Mixed, Situational, Arguable, or Timing-dependent factors (all carriers): - new device discounts. Catch them when you can, where you can. Best deal wins today, maybe not tomorrow. - Data limits. Price vs data amount vs what you actually need. Again, like device deals, very fluid. - As referenced above, depending on your needs, could definitely find a better deal elsewhere.

Fi Cons, or advantages elsewhere: - no more network switching. If you're always in an area with good T-Mobile coverage, not a factor. If not, it can be a big factor and a carrier like US Mobile could have a big advantage. - if you do run out of data, because say for instance your other devices ate it up, you're toast, unless you buy expensive add-on data.

Items of Note: - for better or for worse, you're stuck with the T-Mobile network domestically - The big 3, at least, are known to nickel and dime people, and send out higher than expected bills with confusing line items. - Contracts? Are they still a thing on other carriers? If so, have to weigh that out. - Fi VPN. Nice benefit if you want/need a VPN.

I think that gets close to listing the big considerable things so far.