Portable WiFi devices

If you want just WiFi, way not use the free resources?

A portable WiFI hotspot uses the same technology your smart phone uses to get internet: the cellular phone network. If you are in an urban area you should get excellent signal with either one. If you are in a very remote area you may lose reception with both of them.

However, your phone may not be optimal for use in central Europe - depends on what kind of phone you have (the phone manufacturer, not the service provider). If you have an expensive, newer phone, it should have all of the European data frequencies so should work just as well as a WiFi hotspot. But if your phone is older or more of a budget phone and does not have, say, European LTE frequencies, the hotspot may work a little better. It also depends on your needs.

Example: In May, I took my US Moto E4 to Portugal and used it with my Dutch Vodafone SIM. I got LTE connections maybe half of the time, because my phone has only one European LTE band (band 7). I had 3G the rest of the time - although that was actually fast enough for me. Had I had a more expensive phone, it probably would have had all of the European LTE frequencies and I might have had LTE almost everywhere (although, again, it didn't matter much to me - 3G was OK for my needs). One of those WiFi hotspots might also have had all of the LTE frequencies.

Why not just use your cell phone? One less item to carry. Also, as previously pointed out, free wifi is readily available.

My two cents:
One should not assume that free wifi will be generally available even in large cities. I was in Berlin last fall and looked at a website what showed all of the free wifi hotspots. There were very few. My nephew, who has worked in that city for the past four years, says that it is difficult to find free wifi spots in that city. Furthermore, from what I read, it is difficult to purchase a SIM card for your phone in Germany without a permanent local address. That's why a portable wifi device might work. Check the free wifi availability of the places that you will be visiting before deciding whether to bring along such as device. In Berlin I ended up using an international calling card which was ok for making calls to landline numbers, but I was dinged $10 per call to mobile phones (learned this too late).

Furthermore, from what I read, it is difficult to purchase a SIM card for your phone in Germany without a permanent local address.

Every country has different rules and regulations. Yes, technically you need a local address to buy a SIM in Germany, but people seem to have found ways around that (use the address of your hotel or AirBnB?). And you can always buy a SIM from another country online before you head to Germany and use EU roaming.

That's why a portable wifi device might work. Check the free wifi availability of the places that you will be visiting before deciding whether to bring along such as device. In Berlin I ended up using an international calling card which was ok for making calls to landline numbers, but I was dinged $10 per call to mobile phones (learned this too late).

Ouch. FYI, if you have a smart phone, you can use Google Hangouts or Skype to make calls to regular phones for just a few cents a minute (cell phones cost a tad more in some countries, but nowhere near $10 per call for short calls). And Hangouts lets you call US phone numbers, even landlines, for free. You can do this even on WiFi.