Hi. I'm going on a trip beginning of November to Budapest, Vienna, and Prague and we're thinking of using air BnB. I've never used it before...does anyone have any suggestions on how to tell if a place is good? I've read reviews and they all seem positive on many of them. I've heard some people mention codes before...what are they and does anyone have any we could use? Also we are wanting to go to Salzburg for a day. Has anyone gone from Vienna and come back in the same day? Should we just stay overnight instead? Any insight would be helpful. Thanks.

AirBnB is awesome!! I've used it for places in Amsterdam, Copenhagen & Prague. All the accommodations were exactly as shown in the pictures and the hosts were all great to deal with. It's honestly a lot more affordable than staying in hotels. If you plan to stay in Prague, I recommend you find the hosts Emil & Olga, their company name is ILY-terra. They have several different apartments for rent in Prague. Their prices were excellent & they even offer pick-up/drop-off from the airport. I highly recommend renting from them!!

Oh, and the codes people take about are just so that the hosts know you are a 'real person'... When you go to send a message to a host or make a booking, it will prompt you to enter your cell phone number, and a text will come through with a code to proceed with booking.

I think Dana might be referring to the promo codes that get you a discount. I haven't seen any actual discount codes for AirBNB for a while, but you can get a $27 credit when you sign up, if you use someone's referral link.
So far I've had great experiences using AirBNB in Vancouver, the Azores, Kauai, and Iceland. My only somewhat negative experience was using it in Turkey. That was also the first trip I had used it, so I chalked it up to a learning experience.
This guide explains what I've learned from using AirBNB the past 2.5 years...
http://www.yvrdeals.com/how-to-use-airbnb-the-first-site...
I agree that the reviews can often seem overwhelmingly positive. I think there's a few reasons for that. The biggest one might be because guests are sometimes afraid the host will give them a negative review, if you leave the host a negative review.
My strategy is usually to look for the places with the most reviews, with a rating of 4.5 or 5. (I have yet to encounter a time where I was forced to pick 4 or lower). Any place with 15+ reviews and 4.5 or 5 is a pretty safe bet in my opinion. If anything negative gets brought up more than once in the reviews it might be a real issue.
YVRDEALS.COM
How To Use AirBNB - The first site you should visit after booking your flight