I've used airbnb thru the US and over to Europe...at last count I think I had 11 and have 3 booked for upcoming trips this year, and still have to book a few more.
For me, I like at least 20 or so reviews...if they have a lot of cancellations (airbnb will automatically post a review saying 'this reservation was cancelled so many days before the stay') I tend to avoid it. If they only have older reviews, most likely they aren't doing it anymore. I like some good recent reviews. The place I booked in Paris has 31 reviews - but all are excellent. I booked a place in NJ across the river from NYC and it has 27 reviews...but again...all are excellent. I do find when you do a search with airbnb, it tends to default the more popular places at the start.
Someone else already mentioned the payment structure with airbnb.
I used to avoid the security deposit places, but I don't worry so much now...I'm not going to trash their home. As for a cleaning fee...well, I used to avoid them as well...if you are only going to be in a place for a night or two, a $30 cleaning fee may add a little too much to your payment (that $50 a night isn't so good if it's all of a sudden $65 a night). If you are staying somewhere longer term, it's spread out over more days so it's not as big a deal (and of course I've seen cleaning fees anywhere from $10 to $50). I tend to just go with what the final price is - I have looked at ones that seemed well priced, but then when the cleaning fee was added on (and some can be quite high, especially if you are only spending one or two nights) it put it over my 'budget'.
Make sure you look for 'hidden' fees. I only have noticed it once, but when I was looking for a place in Paris for our upcoming trip, one renter wanted something like 25 or 50 euro in cash when you showed up to cover cleaning costs(!). So you want to make sure you read thru all of their description and house rules - eg...I tend to avoid ones that allow smokers as I don't want to have to deal with the lingering smell. Make sure it has the amenities you want - will you be driving and require free parking? Do you want to be able to use the kitchen? Is wifi impt? Do you want a washer/dryer, elevator, air con, heat? Check where it is on a map - is it really far away from public transport or major sights?
I have looked at VRBO and HomeAway and Cross Pollinate, but I seem to keep going back to airbnb because it is what I am used to. And I'm one of those crazy people who actually has fun looking for somewhere to stay. I looked thru probably 60-70 airbnb places in Paris, narrowed it down to 27...then narrowed it down to 20...then did a top 10 and bottom 10...then a top 5 and bottom 5 out of the top 10...then decided which was the best for me of the top 5 (which had become a top 4 because one of my picks got booked for the days I wanted). Then agonized over whether I made the right choice...lol...
If you find the right spot - especially if it is in a popular city at a busy time of year - don't hesitate. The aforementioned Paris trip...I was searching at the end of April for dates at the end of Sept and one got booked in the week while I was looking. And there is another thing with airbnb - check out the cancellation policy of the place you are booking...some are quite liberal and will refund your whole amount up to a few days before...but others are very strict and will issue no refunds once you book - so if your plans change, you could be out of luck. If you book one with a strict cancellation, make sure you are 100% sure.