They are only CPU limited in some games/applicatations. Any engine that really pushes the Graphics cards, you will see real improvements in. There are some driver issues in some games, but that's to be expected with technology that's not even available to the general public yet.
Also, SLI solutions are not intended for the mid-range user. They're not a gimmick either, they show huge performance increases in many games/applications. Each to their own, anyway, I'm not personally prepared to pay $1000 just for a couple of graphics cards, but some might be.