Having used AirBnB both domestically and for two European trips, I can report that, for the most part our experiences have been good. There are a few pointers I can give when looking at the listings. Be cautious with listings without enough pictures to reveal the details in each room and views from the street is a must. Without getting the exact address before booking it may not be possible to see the neighborhood, but it is valuable to at least look at the area on Google Maps Street View. Looking carefully at the bathroom can be revealing if, for example, you see a sit-down shower/bath, instead of one you can actually stand up in. Some bathrooms have demand water heaters, but if you see something that looks like a small water heater tank, that's probably what it is: you may run out of hot water before you're rinsed off. So, if you have any doubts, ask. I've found AirBnB hosts to be very responsive and forthcoming. Also, be wary if the host mentions dogs. Barking dogs can be a serious problem when you're trying to sleep.
Our accommodations have run the gamut from small but comfortable spare bedrooms with a bath across the hall to full apartments. Ask about the size of the bed. Many places have only twin beds, though occasionally you'll see a queen size; rarely, king. If you're going to have a car be sure to ask about the parking situation. Is it on the street or in a more protected location? Some accommodations include breakfast. Others we've used are self catering, often with some basic and even generous provisions included.
The concierge service of AirBnB is first rate. You can reach them 24/7 wherever they have listed hosts if, for example, you're locked out or have some other emergency. If there is an unexpected cancellation by your host AirBnB will help you find alternative accommodation, even giving some credit to upgrade if needed. That happened to us once out of about 25 reservations. Even then, we had about a month to find an alternative. That's another thing: plan ahead, far ahead.
I think we are now seasoned AirBnB users and we can strongly recommend the service. But, like everything else, you get what you pay for. We have usually been able to find accommodations, even in big European cities ranging in price between $70 to $120 per night. Some of these were self catering apartments, while others were private rooms where the host included breakfast. English breakfasts were uniformly immense allowing us to go until dinner with just a light snack for lunch. In Italy, the typical breakfast was meager, but bringing along yogurt and granola was enough to get us by. For locations where a continental breakfast is the only option, it's useful to see if a refrigerator is available. Some places, though not strictly self catering, did allow use of the host's kitchen or refrigerator. It's also useful to plan a laundry day and book a place that includes a washer. I bring along two lengths of 1/8" nylon cord for hanging up laundry.
So, look carefully at the listings. Ask questions. Expect to have some delightful travel experiences. We're a retired couple with two month long European trips under our belts along with numerous domestic trips and have found that AirBnB is almost always superior to the typical hotel experience. We still use hotels, especially when staying for just one night; on a domestic road trip, for instance. But, if possible, with enough advance planning we try to find an AirBnB host in the location we are visiting. Sometimes, this has proved difficult or impossible either because the places can't accommodate our schedule, or not convenient and we then fall back to a B&B with good recent Trip Adviser ratings. Works for us.
On last thing: If you need to confirm or discuss details with an AirBnB host you can usually call them. Be sure to take into account the time difference. And, Skype makes calls to European phone numbers cheap: about 7¢ per minute.
