I had the infamous P0420 code on my 2002 Outback (bought used in 2009 with 90,000 miles). Started in 2012 with about 130,000 and was intermittent for about a year, going off for a few months then coming on for a few weeks until in late 2013 the CEL was on all the time. Luckily I was able to pass NY State emissions before the light was on full time. Subaru told me the cat needed to be replaced and they'd be happy to do it for $1,700. Another local shop told me they'd put a Chinese cat in for $650 but it would probably not last longer than a year.
I decided, after reading everyone else's horror stories here on Outback.org to replace both O2 sensors myself. $100 for the rear sensor, $60 for the front one. I also bought the claw foot tool. However, since I really hate getting underneath a car, my brother told me about this guy he knows down in the rough part of town who has a repair shop. Went down there with my brother and the guy asked, "did you spray them with WD40 yet?", I said, "yes, I did last night", he said "80 bucks to change both". I said "ok great", but I really wanted to jump for joy. The guy later said O2 sensors get "sluggish" and then fail and will throw a P0420 code. He said it's usually the rear sensor that should be changed first. I have 152,780 miles on my OB now.
Came back to the shop an hour later, the job was done. CEL has been off for over 300 miles already, passed emissions. I think $240 is a lot better than $1700 or $650, don't you? Sure the CEL could come back on at anytime, but these dealers and shops don't care about you, they just see you as an ATM machine. This website is one heck of a resource for DIY or finding out options to really expensive repairs that might not be necessary.