Like Eric stated, all synths will lose loft, some faster than others. Primaloft One gets real thin really fast--even the newest generation. However, I still find my primaloft garments pretty warm, and they still insulate well even though there is a noticeable loss in loft. Other than weight the disadvantage, there's also the compressibility disadvantage. Synthetics don't compress as well as down, nor would you want to compress them as much (again, concerns over loss of loft). But of course, they still provide insulation value when wet, and are easier to clean in home washer and dryers than down.
Hmm loft life depends on use. The back of your jacket, or bottom of your sleeping bag will definitely be the first things to go.