As safe as any tourist-oriented destination (ie the usual stuff about valuables and being sensible apply).
Sousse was a lot like the Red Sea resorts during the Egyptian revolution.... while Cairo was a hotbed of activity, it was pretty much business as usual in Hurghada. Same applies to Sousse.
be aware though, that it's Ramadan at the moment, so if you travel outside the resorts into more traditional areas of the city you'll need to be respectful of the customs. almost 7 years ago