I bought Spline Patch some time ago, and got Splurf recently. While Splurf is a lot like Spline Patch, I thought it would make the process easier because of its Spline Select function so I gave it a try.
I tried to model a Shelby Cobra using splines, and both plug-ins. Here are my conclusions:
While the idea of using splines to make complex models is attractive, it is nonetheless counter-productive. It takes a long time to build the model with splines, and control is not easy because you must work with points on individual splines only. You can’t work on several splines at once (in point mode). So it takes a long time to get something that works.
Then the problem is to make the polys using either plug-in. For some surfaces it works well enough, but as it gets more complex it brakes apart quite easily. Connecting all the patches in a multi-patch or connector patch does not always give very good results either. Also, you can quickly end up with a long list of splines and patches to go through in the object manager.
It is my opinion that there is very little use for these plug-ins, and modeling in polys is much quicker, much easier, and more straightforward. I would not recommend Spline Patch or Splurf at this point. They do what the developers advertise, but as I said before, it is a slow and difficult process compared to polys.
Others are welcomed to challenge this comment of course.