i remember seeing a post a little while ago about a user who had ncix build them a system. it wasnt working properly (cpu temps were too high) and they took it back and fixed it without issue and gave him some free service (basically additional wire management) as well as a free service call for his other (non-ncix purchased) computer.
i've seen plenty of positive but havent heard any of the negative until now.
you will get negatives with any company. i've pushed over 10k through newegg and i hear all sorts of horror stories all the time yet i find them perfectly fine to deal with. i've heard horror stories about microcenter yet i personally love the store. ebay, best buy, target, sears, barnes and noble, amazon.... every single store has good and bad.
personally i've never dealt with them (i deal with newegg) so i cant give you a firsthand opinion and can only let you know what i've heard and my general opinion.
as far as them building a computer.... they do have pretty good prices (and upgades arent crazy expensive like most pc retailers) and they list every single component so you know for sure what is going into your system. no hidden bad psus, cheap ram or subpar parts slipping by unnoticed its up front so you know exactly what you are buying.