No real downside, but it is still toxic-smelling and you'd need to do it either outside or in a garage with your big door open (how I do it). I mean, it is advertised as low odor, and it is compared to the regular fixative, but I'd say not to spray it in a closed room. It is very quick drying though. Leaves a somewhat rough surface on the paper, so it will never smudge after you use it. You can't add more pencil afterwards though--because of that texture. Make sure you use it when you're finished.
I usually do two light coats. You can coat it a second time in 10 minutes (to be safe), but it dries so fast. Do not overspray it. A light spray about a foot away will do it. Not needing much of it cuts down on the odor too.