The purple guns from HF come in different variants. The cheap one is on sale typically in the $12-20 range, and the more expensive one is $30-50 and may come with a regulator guage.
The reviews on both are mixed, and like anything, appears to be more about prepwork than the tools. I actually bought the cheap one this weekend for epoxy primer, latex paint around the house or anything else I wanted as essentially a throwaway HVLP. I also bought the 2 gun automotive set for $50 on sale.
There is a definite build quality difference. Trigger action smoother, whole feel of the gun is better balance and just feels more refined and better built on the more expensive gun.
I did read the reviews about disassembling and cleaning them first, and sure enough, I found the threads on both had metal shavings from the thread cutting operation in the threads. The cheaper one had the nozzles partially clogged by metal debris as well. If I had attempted to shoot either one before cleaning, the results would have been terrible. A quick cleaning of the threads, orifices, and general checkup resulted in two guns with much better adjustability. I haven't run fluid through them yet, but already they work better just in turning the threaded portions, and the air blown through is predictable. I need to run some colored water (water with food coloring) to test the patterns.
Overall, not bad. Not fantastic, but I think I can make them work. It's all about knowing the tool before you use it.