Took a look at the Smallville RPG today and, while it looks nice and probably plays pretty well since it uses the Cortex RPG System, I don't think this RPG will have "legs", simply because it's based on a media, specifically movie or TV, property.
Taking a look at the store shelves, I see role playing games for Buffy the Vampire Slayer, Angel, Serenity, Battlestar Galactica, Supernatural, Dr. Who and, in the used section, Men In Black, Hercules/Xena, Star Trek, Star Wars and StarGate, all of which had varying amounts of support and, with the exception of Star Wars and Dr. Who, haven't sold in months.
Hoever, RPGs with licenses derived from books, tend to sell steadily. We still sell both the older and newer DC Heroes RPGS, as well as the Marvel based ones. The Elric RPG sells, albeit slowly, as does Call of Cthulhu. The reason for this, based on my experience and research, is that the book licensed RPGs continually have new material coming out. In the case of the comic based ones, there's a whole new crop of material ever month. Michael Moorcock keeps putting out material in his Eternal Champion series and a month doesn't go by without a new Cthulhu based short story. Among the media based RPGs noted above, only Star Wars and Dr. Who have fresh material coming out on a regular basis. Without that new material, they would suffer the same fate as the others in the list.
The Smallville TV series starts its final season next week. Even with the announced list of products shipping from MWP, the track record of similar media based RPGs is pretty dismal. If MWP had gotten the license for the game earlier, much like Eden Studios did with the Buffy RPG, the company would have stood an excellent chance of selling a good quantity of the books. this late in the series' run, I don't expect the RPG to last much beyond 2012.