I'm way late to this thread, but since I stumbled on it through a Google search, I thought I'd add my own $0.02.
I've taken my Droid to Italy and Costa Rica. Both times I kept it in airplane mode but turned on wifi and GPS. I only tried using GPS w/ Google Maps while I was connected to wifi, and that was very hit-or-miss. I'm heading to Europe this summer for a few weeks and I'm going to try out a few different GPS-only (no maps) apps there.
Google Voice was *great*. I didn't try calling out over wifi, but whenever I was connected to wifi, I was able to send and receive texts with no problem whatsoever (although for longer conversations I used GTalk, since GV does have that 5-minute refresh thing when sending texts.) Voicemail was really nice to have. My apps ran without a problem. The best thing to me, though, was the camera--I was able to take tons of pics, do a bit of in-phone editing w/ PicSayPro, and email them to my Flickr--it'd store the unsent pics until whenever I had a wifi connection, and then it'd send 'em all at once. I really loved it--in fact, I brought my laptop with me, but the only time I really took it out was at the airport, and to offload pictures from my DSLR.
Another bonus--when at US airports and hotels, I used my phone to tether so that I didn't have to pay for internet. When I was at a hotel in Costa Rica, wifi cost, so I paid for it for my phone, and then the one time I wanted to use my laptop to upload some pictures, I tethered my phone to my laptop so that I didn't have to pay for a second connection.
I love my Droid.