I've had Rykas before and they weren't a great fit for my feet, but lasted for ages (took them travelling overseas and they withstood a whole lot of punishment!!!). My friend loves them, she was fitted for them at Athlete's Foot and was a bit dubious at first, but is now onto about her third pair. I think hers last 6-9 months, but then she uses them most days (lots of running).
I've got Asics Gel Nimbus at the moment and am coming up to 6 months of use and am starting to notice them being less comfy than they were. For me I figure if I'm pounding the treadmill or the pavement for an average of an hour a day with my weight hitting them, they're going to wear out!
Ali