They're great for the "out of the box" experience of being to do a job quickly and easily, but experienced people will be able to do the same job as well and probably better using simpler and cheaper tools. I love the Stewmac tools like the tang nippers etc. purely because I don't have that experience and I'm not patient enough to gain it! My good friend (and local folk blues hero) Karl Svarc does all his setups and general work using traditional simple tools and a LOT of patience. Despite Karl's modesty, he can set up and dress "manageable" instruments and make them play as though they are much more "upscale in worth". Despite being more than capable of doing it myself, I would rather Karl set up and dress my builds (and teach me as well!) than me doing a halfway decent job and learning moderately little.
I'd say buy them if you can afford them. You get quick and predictable results, but it's worth learning, understanding and trying your hand at using traditional techniques. As with anything, you develop your own style of working that way :-)
Oh yes, I have to point out that the majority of Stewmac's tools aren't actually custom from the ground up, a lot of them are simple and easily available tools (if you know where to look) which have been modified to suit a specific purpose. The tang nippers are available elsewhere for much less, but just need a groove milled into the seat. The fretwire nippers which cut flush to the board can be made using some end cutters with the top face of the cutters milled flat instead of bevelled. Two minutes on the spindle sander grinding them sorted them out!