A net is not a replacement for field work, but a supplement to it. If you buy some new discs and want to get a feel for them, go to the field. If you want to work on your throw, a net may be the trick. I'm working on my left handed throws right now, and the net is perfect. I still have about 1/5 throws that aren't anywhere close to where I want them to go, and I'm trying to eliminate those. The cage I throw at has proper dimensions that the disc will hit the wrong wall/ceiling if I throw one that badly.
Just like there's no replacement for seeing a disc fly, there's no replacement for 300 throws/hour.
Also, with a little imagination and prediction, I have a good idea where my disc would have ended up.