I just changed the upper guide rails and chain on my 87 420 this weekend. Two of the upper guide rails had broken off at the top, and the chunks were riding between the chain, one on the left and one on the right. I was definately driving on borrowed time.
This was my first time chaining the upper guide rails and tentioner, so It took me a day and a half to pull everything off, deal with stripped 6mm hex nuts, and pull the pins. By contrast, putting the new chain on took only half hour with a helper turning the engine and advancing the channel locks. By all means do the chain if your gonna spend all that time chaining the rails.
www.pindelski.com has a great pictorial on changing the rails and chain and is worth the $30 yearly subscription fee for his site. My indie in San Francisco wanted $1600 to do the job while I did it for $225 in parts and a sore back.