I rather like them, especially for citrus marinades. It's very juicy and moderately aromatic. I like makrut and finger lime aromatics better, but they're not juicy enough to make the marinade. Lemons are another great option, although I think it can easily outweigh the lime flavors.
I'm planning on growing some from seed as rootstock because of their extreme drought tolerance. I've read crops on Rangpur rootstock can handle up to 240 days without rain!
The "fruit quality" (usu. measured as sugar/total soluble solids) is moderate, so other rootstocks will probably yield sweeter fruit, but for sour citrus it's probably a great choice.
Calamondin taste is very good but rather different. I'd say grow both!
Also, in addition to FourWindsGrowers, you can get seeds in bulk from Lyn Citrus Seed, and you might have luck finding it on craigslist. There are a handful of people selling Rangpur trees currently.