So I've been building wheel for over 15 years, every since I started racing in my late teens when I didn't have the money to pay someone. I've built for other racers and friends so I have experience with a varieties of rim. I'm not a pro builder, I've built around 50 or so wheels. So recently I picked up a pair of Alex R390 for my son's bmx race bike. Just out of curiosity I google'ed for "Alex rim review". I found the typical Mavic is good and a mix of responses for Alex rims. What I don't find is real quantified qualities that make a rim good or bad. Most responses where related to the wheel as a complete unit. The wheel went out of true, won't stay true, etc.
So giving similar rim, for example a 32h with similar box, double eyelet, welded, machined brake surface how can you quantify rim quality. Over the years I've build numerous wheel sets with a variety of different rims but all of similar make up: 32 hole, typical box section, eyelets, both pinned and welded. Can't say I've really noticed a difference.
Comparing two rims with similar features the only thing I can think of is the aluminum quality and maybe the box structure. But then my experience seems to show most "quality" rim are more then good enough to survive a season of racing. I've had to replace rims mid season due to problems but those problems have mostly been with a quality "french" company.
So all you experience wheel builder what metrics do you use to justify one rim over the next. Have you notice significant difference in the quality of a build between two rim. I'm not looking for Mavic versus Velocity, versus Brand X but what about a rim have you noticed that makes it a better rim.