I think it will be linear, and while I'm not a fan of strict linearity, a heavily-scripted game can still be fun and enjoyable to play. Just look at Call Of Duty, Max Payne or Half-Life - which are so on the rails linear you can almost see the tracks under your feet. The original Dark Forces was also very much a linear experience - in fact, most FPS games suffer from this particular complaint.
In my view, they should design levels with multiple pathways to reach an objective. You can still have scripted sequences via each route, and still place the appropriate markers on objects/locations for your team to use -
while at the same time offering a bit more choice to the player.
This game isn't trying to be something like Ghost Recon where you plan routes beforehand, and micro-manage your team - and I'm glad, because I'm not a big fan of strictly 'tactical' shooters. While you might have more freedom in the terrain with those types of games, they still have scripted events kicking in that you have to work through.
Anyway, if the level design itself is well executed, and the gameplay and 'story' is compelling, then I can put my dislike of linearity on the back burner while playing RC. It may not initially have a high replay value, but if the gameplay in SP is very well designed, then I will probably replay it some time in the future. After all, I've done that with a lot of my games already, linear or not.