For starters I doubt we will sign Roberts. I think he will end up at Souths.
As for should we be in the market for him. I have mixed thoughts on the matter. He certainly is a strike weapon. His speed can lead to tries from anywhere on the field. He would add some much needed speed to our backline. He is probably better than Lomax and Lafai. He is also much better than either of our wingers, but I doubt he wants to come and play wing for us. Wing is the position I would play Roberts right now, instead of Ravalawa. I would also drop Lafai for Roberts if I had to make a choice. The downsides are, I think Mary would certainly drop Lomax for Roberts in a heart beat. His game is a bit one dimensional. He does lack a passing game and doesn't set his wingers up, as well as he could. He has also be troubled by injuries this year. He wouldn't come cheap either.
I would probably sign him if I could.