I am surprised that there have been no 'they are horrible; destroy your rifle' type posts yet because that certainly is the impression given by many people. I have seen, and shot many rifles fitted with square section springs; they may have been Ox brand (are there others?) but generally speaking the rifles shot very well and I could see no sign of any damage caused by the spring. A few were 'harsh' but on inspection they were clearly running very close to or over the limit. No doubt the previous owner had bought a replacement spring that turned out to be a square section one and crammed it in with no thought to power output as they most likely had no chronoscope. I have also seen an inappropriate square section spring fitted... clearly the wrong size, too great od such that it began to bind in the piston on cocking.
So ... I am not sure they deserve the bad reputation they seem to have built up ... but personally whenever I am replacing a spring I will use a round section one.
Cheers, Phil