Just been round the Four Abbeys Cycle Route in the Borders. It is 55 miles and takes in Melrose, Dryburgh, Kelso and Jedburgh Abbeys.
The route is mixed taking in dirt track and busy A roads with everything in between. Signage was also rather mixed. To begin with, leaving Melrose it is excellent and even has the occasional sign just pointing straight onwards to reassure you that you are on the right track. On leaving Kelso though I found myself off piste by following the sign onwards when I should have turned right. A couple of places even had signs pointing left when you should go right and one turning didn't have a sign at all although there is an alternative route which is longer.
There are some fairly long climbs but no horrible gradients and since I did it on a road bike with panniers (the trip was organised when I still had my MTB, sniff) I should know. There is a pretty long loop to get from Kelso to Jedburgh but most of that can be cut out if you wish and just requires a little back tracking to get to Jedburgh. The site I linked to recommends taking two days to do it to allow for maximum sightseeing.
The busier roads could have been potentially unpleasant but I found that drivers were very good and waited behind until it was safe to pass.
So I would give the route a big thumbs up since it is very picturesque and great cycling. And the abbeys are lovely. I would recommend either taking a GPS and paying close attention or checking your map at turnings after Kelso due to the interesting signage.