I've had stock shocks that have never leaked (7k on these) and I've had top drawer shocks that have leaked. Many of us right here on this forum had the Progressive 440 shocks leak. The good thing about it is as long as you have your receipt, Progressive will rebuild them for you.
Shocks are one of those things with many moving parts with fluid behind them.....some are going to leak.
Russ, my experience is the first thing you notice is the shock oil all over the shock (and often the bike). It's still rideable although it may be a little rougher ride. Some people don't notice any difference.....I've pointed out to two members that their shocks were leaking. Sometimes, you'll just happen to notice it before they do.