Throughout my career I have never seen an RTM version that was substantially less stable then the following SP1. Sure, there were bugs and issues. Sometimes there were critical bugs and issues. But there were just as much bugs and issues in SP1 and in SP2, and so on. I haven’t conducted a thorough research, so I don’t have a statistical proof, but these are the facts, at least from my experience.
I’d add one more thing: pre-release versions of SQL Server run in production as part of Microsoft TAP (older link, and I think RDP and TAP have merged together at this point, but I don’t have those inside details). These are real companies with real workloads running pre-RTM versions of SQL Server. I work for a company which is in the program, and we were running our data warehouse on CTP 3 and then RCs. By the time RTM hits the shelves, there’s already been a good deal of burn-in.