I bought one of those stir starters for $40, online. Sure, I could have built one from magnets and a project box, etc., but at the time I had some other DIY brew projects going on and was struggling and getting frustrated that things weren't working as I expected so, to spend 0 hours working on something and have it work out of the box was worth it to me.
To answer your question, in my brew set up, a stir plate is 100% worth the investment. I'm not sure it is any faster than not using a stir plate, but you definitely yield more yeast. 20% is significant. Especially, when I brew 11g split batches and only need to buy 1 tube of yeast and can get it all done in a 2L flask for most batches, if I do a bigger beer I might have to step it up, but no biggie because I can get it done on the stir plate.
Fortunately, stir plates aren't one of those things you "NEED" to make good beer as is fermentation temps, healthy and adequate yeast pitch, etc. Rather, it is one of many solutions to get you to the goal of pitching enough yeast. There are alternative: Buy more yeast, brew a smaller beer and harvest yeast to brew a bigger beer, etc.
A stir plate is something you should probably wait until you are prepping for a brew day and say to yourself, "I'm really at my limit for yeast propagation, man, a stir plate would really help right now!" When you find yourself saying that a few times, then its time to buy a stir plate. That is how it went for me, as I started with the walk by and shake method. But when you start playing around on yeastcalc.com and mrmalty.com and realize how much more yeast you can yield using a stir plate you might chose to buy one.