I've played with spindle sanders a little (though never on a handle)....and I like them for any kind of curved work. That said, with so many things to capture my time, dollars...or limited workspace, I don't have one. I think of them more as a convenience than a workshop necessity. Definitely not in the essential woodworking tools category for me. That might be different if I was churning out a lot of stuff.... but for the occasional handle or curved piece, other methods get the job done quickly and easily enough.
Don't want to stray too far from the question but, if alternate methods/approaches are of interest for getting coke bottle shapes and other complex curves: I like to rough out curves (if the project allows) with a bandsaw and then finish the shaping with hand tools. I'll use a mill file if possible because it cuts fast and smoothly but my real trick for even curves is to cut some wood strips at about 1/4 thick and put coarse sandpaper in the middle (either adhesive backed sandpaper or applied with some type of glue that won't leach through the paper). I find these sanding sticks get the job done fast for shaping. You can cut the widths as needed and you can bend them along the radius of many curves. The amount of pressure you create by bending them helps to vary or control the cut. And, as needed, you can easily shift to narrower or wider pieces to get the results wanted.
as for consistency, to check the work along the way and make sure my shape is consistent from side to side or top to bottom, I'll make a flat template of the curves I want and then I can compare that, and each side to my work in progress with a contour gauge.
I figure, in the rare case I need a spindle sanders services, I can always chuck a sanding drum into a drill press .
Here's a very quick crude thirty second effort to render a picture of what I mean by the sanding sticks if the explanation is not clear. It's a head on and a profile view ....