Peter, a friend of mine, recently bought a used A&T 3GS from another friend. Since the phone's contract had passed AT&T approved a request for iTunes carrier-lock removal. So it was no longer a carrier-locked phone.
Peter took the phone to Europe and did very well with it there. I've not read many success stories so I'll pass on what he emailed...
Traveling to Europe the past several years I've taken along a very basic 2007 era unlocked Nokia GSM cell phone. It was always very easy to find small mobile phone stores. I would just walk in and buy a SIM card for that country for about €10 or so. It usually came with some calling time credit (easy to buy more credit at phone stores and some local shops), and the young guys staffing the stores were always helpful with setup assistance.
Buying an in-country SIM card allowed cheap calls and texts within in that country, and more expensive calls out of the country. Keeping in touch with friends and family back home can also be done very cheaply, if calls are made from the US to the European network mobile phone by Google phone from a computer. These calls from the US to a European mobile phone will cost about 10-15 cents per minute (calls from a computer in Europe using Google phone to a US land line cost less than 10 cents per minute).
[Peter then acquired the iPhone ...]
Setup of this clean iPhone with factory settings was easier than expected. I didn't have a SIM card in the US, but the setup was very simple using my home wifi network and following on-screen prompts. During the wifi setup I easily register the phone under my name using my Apple ID number, and synced my Gmail contact data (since I had previously added phone numbers to my Gmail contacts, I now had both email and phone contact data on the iPhone). I then hooked the phone up to my MacBook (OSX 10.5.8) and synced with iTunes. I enabled iCloud for contacts, email and Photo Stream and then bought a few apps. Very easy and intuitive.
In Rome a few days later near my apartment I found a small neighborhood mobile store for a major Italian carrier (TIM). As usual the techy guy working there spoke English. The process was simple - I showed him the iPhone (though he was a little surprised that I had an un-locked US iPhone), and asked to buy a SIM card (€20 with €15 voice and SMS credit plus free unlimited data for 1 month). He then had to register it with my passport info and do a little setup. The whole process took about 30 minutes, and with a little lag in activation time I was up and running with iPhone voice and data service in Rome within 1 hour.
Note: iPhone 3Gs uses a regular sized SIM card, iPhone 4 and 4s use micro SIMs, which may be harder to find in Europe (though it is possible to "cutdown" a regular SIM to micro size). Also you can buy a new basic unlocked GSM phone in Europe for about €30.
During my 2 week stay in Rome, the iPhone was very helpful - calling local friends, restaurant reservations, email, camera, using GPS and maps for locating sites and restaurants, and finding walking routes between sites... Especially helpful purchased apps included: Italian dictionary and verbs, Rome2Go, Rome Travel Guide - Lonely Planet (linked to GPS maps this was incredible useful), Contact Sync for Google Gmail, TripColor and Kayak Pro.
Moral of this story: When traveling in Europe, an un-locked iPhone is a fantastic asset. Next best would be any unlocked GSM smart phone or basic phone.
I'm amazed that a 20 euro card came with unlimited data!