I do feel adfly's a bit so-so in terms of security-- but a fair source of revenue for server costs. It's a bit worrying how many malicious adverts get through; most I've seen during my accidental clicks were explicit or a bit dubious in nature. Then again, the worst malware I've received was from deviantART; all sites have the potential to throw up dangerous advertisements, no matter how reputable they are! It's just the nature of the 'net.
It's definitely a legit site that's frequently used across many, many forums-- but it undeniably does have the potential to pose some security risks; it's very easy for something malicious to sneak by. Clicking on one of the adverts pretty much guarantees your computer's demise (or a link to a very dodgy survey), too! I'd rather there was a donation button, but that's just me!
I'd suggest either avoiding the links, or using some from of anti-script browser addon (I use ScriptSafe for Chrome) in conjunction with AdBlock when it's necessary to click on one. It's a frequently, oft-used war to generate money, but similar to all sites, not all advertisements are safe.