I happened to travel to Ireland shortly after T-Mobile announced their Global Data feature, and overall it was mostly a pleasant experience. There's certainly a convenience of not having to get a local SIM card or contacting your carrier to arrange service overseas, but just to land and you instantly have 2G internet service.
Is it fast for data? No, I'd certainly not make that claim. However, it works perfectly well for navigating with something like Google Maps, which uses data-friendly vector maps for most of the data downloads. If you're looking to do something like stream music or Netflix, 2G-ish speeds will simply frustrate you, but if you just want to tweet, Yelp, email, and do other mostly text-based services, it's perfectly acceptable for speeds. Uploading photos is doable, it just takes a couple of minutes if you want to send up a high-resolution picture.
I did notice that it wasn't actually using EDGE for data services, but was instead actually using HSPA/HSPA+ for data, and just sending the data at slower rates than the theoretical network supported max. Looking at my speed tests, I was getting fairly consistent 0.15Mbps up/down with 200ms ping times. For reference, 0.15Mbps is about 18KB/s, which is roughly three times the theoretical max of 53.3kbps dialup.
If you're going between a lot of countries, the T-Mobile service is probably worth it as you can use your SIM throughout Europe rather than having to figure out which European carrier will work best for you. If you plan on staying in a single country for a week or more and want some faster speeds, it'd likely be a swell idea to consider getting a plan through a local carrier.
Finally, you can view coverage information and maps for each country here.
The only issues I really found was that a) I had to manually find and register my Nexus 5 with the local carrier network. I suspect this is because I was running a rooted phone with a custom ROM that didn't have T-Mobile partners programmed in, and b) the high-speed data pass option never appeared on the T-Mobile website as an option.