You're in luck! I can comment on all three!
First of all, the Stag Arms lowers are excelent. I purchased one for a project and the fit and finish of the parts is great in the lower is great. Funny thing is, I used a Model 1 Sales kit on the Stag lower. Put a pistol together and all went well. Sight issue w/ mating the upper and lower, but once things broke in, it was fine. M1S parts fit into everything, no issues have come up.... other then the pistol recoil spring is weak (issue has been resolved.)
Cavalry Arms and the Cav 15. I have a number of those lowers (the MKI's, only have one MKII). My experiance with them has been excelent. The early MKI's had few issues, ie a tight buffer tube could bind the carrier w/ certain uppers. The MK II's resolved the issues. I haven't played much w/ my MKII lower, but everything looks and fits fine so far.