From my experience, yes it is very safe to use hospitality exchange services all over the world as long as you check references, have a backup plan, and trust your instincts.

I've been CouchSurfing since 2007 in more than 30 countries and have only once had a less than stellar experience. I've actually had more (and worse) bad experiences staying in backpacker hostels than while CouchSurfing.

Some Tips

Check References

They're there for a reason. Everyone has to start somewhere, but these networks are so big that most people have a friend who can vouch for them already. Check second degree references to re-assure yourself that they are valid. If you're worried about being hit on by a host take a look at the sex of the people they host. Sometimes you will notice a trend of guys only hosting girls and that can be a red flag for some.

Have a Backup Plan

Things go wrong. People cancel at the last minute. You should always be prepared to find a new place to stay.

Trust Your Instincts

If you're getting uncomfortable in a situation, leave. Don't think twice about it, especially if you feel unsafe.

If you're still concerned, I recommend you travel with a friend.

When i was traveling and posted on the couch surfing forums about my travel plans I was messaged by some girl(!) who was very insisted that i drop by on my way. Later she started to make moves on me. (Can't really blame her, i am often mistaken for a young George Clooney.)

However my other experiences where always good. Usually got hosted by people who live alone and just want to exchange travel stories or talk to new people, or maybe because they live in a small town where not much happens entertainment-wise. I usually cross-check the name+city, email, phone number, nick on Google or have a look at the neighborhood on street view to get an idea what sort of person and environment it is.

I would not give too much about the user reviews. This is because, if you had a bad experience with a host you would not give him a negative review, or he might give you a bad review in return and rather just don't rate the other user at all. Maybe if you work out the percentage of reviews vs. # of hosts/visits he had, it can give you a better perspective.

Fun fact: statistics suggest that you are much more likely to die because of diseases like heart disease or cancer than by unintentional injuries such as car accidents or hostile couch-surfing hosts.

So my advice would be: Go couchsurfing and be nice and respectful, but learn CPR and always wear a seat belt and suncreen :)