Please pardon the newb question here, but how do you know if your machine will benefit from Smooth Stepper?

If you have problems using the parallel port or need a faster pulse frequency then you will benefit. If your machine runs well then you probably wont benefit.
 As an example my mill ran fine with the PP and the only difference now that it has the SS is the motors sound sweeter but there is no performance gain.
 The lathe however was needing  a higher pulse frequency to get to its potential, previously I had 1:2 electronic gearing (which I dont like) and I could only get a max of 3300mm/min, now with the SS i have electronic gearing cancelled and can run 20,000mm/min although I have slowed it down to 10,000mm/min.
  The Coilwinder I made ran on the parallel port but didnt sound happy and was a bit jittery,  with the smooth stepper I could crank up the feedrates a bit and the motors run much better.
Hood

Hood did a great job of explaining the performance benefits. You can get an idea of how well your PC will work with the LPT by running the driver test. If you have good results then that is half the battle. Some PCs just won't let you drive the LPT that fast though, even new PCs with an add-on PCI LPT card.

The other major benefit I think is the complete lack of needing to fiddle with getting the LPT working right. I have been pretty lucky as most of the PCs I have tried have done OK driving the parallel port. Some folks really have a tough time getting it going though. With the SS it will work the same on any PC, and you can use an older/slower PC. I use a 1.5 gHz machine in the shop. It runs Mach and CamBam just fine (at the same time even!) and was a $50 investment.

Your machine will not benefit should you need backlash compensation as it dosen't work when using the SS.
( Sorry, only real gripe about it, and over a year old one! ).
RICH