I think it makes sense to start with the HSU and then see if it needs to be supplemented with shakers. To answer your question, shakers are almost exactly like a sub in terms of both feel and control. You typically have a crossover control (usually set lower than the sub) and a level control just lie a sub. It feels exactly like a sub, but it's more powerful without filling the room with sound. As I mentioned, I had my sub accidentally disconnected for a while in the theater room and didn't notice it because of the shakers.
With movies, the key is to make sure the crossover is set exactly right -- usually somewhere around 40-50hz -- you want it to capture just the effects that should legitimately cause shaking. If crossover is too high, they will shake for things that shouldn't induce shakes and become annoying, but when you have it dialed in exactly, it's pretty damn amazing -- and you can control the intensity level from subtle to pretty intense. The term "shaker" is a bit misleading -- if feels like the deepest sub ever rather than actually shaking. In my opinion, shakers are probably the biggest bang for the buck (pun not avoidable) affordable upgrade for movie use. I think you're correct to evaluate the HSU first and then determine if shakers can supplement that.
