I haven't tried the Rivendell bags but i do own Carradice (pretty much the same concept but a lot cheaper!). The Carradice really are amazing, i have the Pendle bag that attaches to my Brooks and i love it is great, doesn't smell when wet, bulletproof, waterproof, handmade in the UK by a family owned business and stylish. SJS cycles is the cheapest place and great service too.

If i would have to find any fault with the Carradice i'd say the only thing is that the older style bags (which i like the best) have attachment and closure with litle leather straps and buckes, very sturdy and old school but not convenient or quick. Very simple modification took care of that problem on my bag. They also offer a variety of more modern/quick release type bag.