I have been learning submission grappling for a few years now. Having used extensively the Knob, the Tiger Tail, three foam rolls, tennis ball and the Stick, I feel that I can compare those products.
As everone reading this probably knows, those tools are used to soften up tight muscles, which can cause stiffness and pain in short term and injuries in long term. I found that the Stick works better for medium sized muscles. Specifically, the Stick was excellent for the back of my neck, my front shin (muscle outside of your shin bone), the forearms (both the back side and the front) and surprisingly, for the bottom of my feet. It was decent for my quads and my calfs but I preferred foam roll for those and other large muscles, including shoulders, glutes and IT band. The Knob worked better than either for small muscles, such as the part where the neck joins the shoulder, around the knee and front part of your body where your leg just joins your body. Tennis ball worked better for glutes, lower back and between the shoulders than any of those products.
So do I think the Stick is worth getting even if you have one or more of those above "massaging" products? Depending on your sport and pain area, absolutely! The areas where the Stick works well (forearms, neck, front shin, and foot) it works better than anything else I have ever tried. If your sport taxes those areas or if you feel pain or tightness there, the Stick will beat anything else.
How does the Stick compare to the cheaper Tiger Tail? I think it is a personal preference. For me, I like the cool and softer feel of the Stick. Tiger Tail is basically a bit stiffer (less bendy) and shorter. I didn't like its plastic foam wrap feel (it feels like cheap bicycle handles). If you like one as much as the other, then you should obviously get the cheaper Tiger Tail.
Finally, is the Stick worth the $40 price tag? I debated for about three months because I felt $40 was ridiculous for what is essentially a HVC pipe wrapped in plastic rings. How much did it cost them to make it? $3, maybe? But after using it extensively, I feel that it is a sturdy product that may last a many years, if properly cared for. So while expensive, if those areas I discussed is giving you any pain, I think it is worth buying.