After conversing with my cell service provider (Credo Mobile) I decided to keep my US SIM card for a 2-week trip to Ireland and Scotland. It worked just fine. I had to notify Credo and have their tech support folks make some changes in my phone ahead of time, then change it back after I returned, but this was all invisible to me as user.
To avoid the oft-cited problem of astronomical data use charges, Credo simply disables your phone's data function for overseas. You can only use the Internet and email when you are on WiFi. Since most hotels and restaurants and even public buses have WiFi, I found this to be no trouble at all.
The per-minute charge for cellular phone calls is something like $1.70 when you're outside the US. Obviously this could be expensive if you have to make many calls or lengthy calls. I avoided doing so.
As a result, my cell phone bill for the trip was less than what I would have paid to Irish and UK providers for local SIM cards, and I avoided the inconveniences of (a) having to spend precious vacation time going to a phone store to get a local SIM, and (b) having a different phone number in each country. The latter is what I really hate -- one reason for having a phone is so that people can call YOU, and how are they going to do that if they don't know your Irish or UK phone number?
Another great inconvenience I avoided was that the phone worked upon arrival in the US so I could call the person picking me up, who was waiting in the airport's cell phone waiting parking lot. With a foreign SIM card this would not have been possible. Last year when I had a UK SIM, I had to find someone in the airport willing to make the call for me.