Is it worth it?

I think it depends on whether you have time on your hands and somewhere convenient to work.
I did 3587 early this year, I thought I had plenty of time to work on it, then business picked up and it took me ages.
I followed the drawings in Keith Callaghan's website pretty much, using some good blocks I got cheap. I'd been collecting the blocks on eBay for ages. It still cost a fair bit in fittings, but then the boat was 11 years old and needed a refit anyway.
If you had a nice big workshop and a good boat to copy, it would be very much quicker.
As it happens I am probably about to put this boat up for sale.

Some boats have semi-onestring systems, or compromised systems that do not give as much variation in rake. Mine will go from leaning forwards a bit to moving the masthead back over a metre. There seemed to be some trial and error, and six months on, there are a couple of bits needing a minor fettle. 

I would say the one-string is worth having, provided you can find time to think about rake in between worrying about tactics, shifts and not hitting things.
Other things to consider include getting good enough sails, and depending on where you sail, a nice lifting rudder. Sails can be bought used more easily than in many classes, but secondhand winder lifting rudders don't happen, a new one is 10% of the cost of a MK1 Winder....3587 happens to have one :-)

Yes, I would do it again if I wanted a GRP Merlin and that's what the budget dictated.
OTOH, I now have a better understanding of why people buy new boats or get them professionally sorted!

Hope that helps?

Linking the lowers in to the jib halyard is a good start and gives you 75% of the one string functionality. You get the same amount of travel regardless of how you rake be it one string, half and half or conventional, the advantage is the more you can get automated the system the less faffing it takes to get there.

The full on conversion is a bit of a fiddle and it is expensive to "have done". I did it to 3583 and it was worth it, however I did it all myself.

I did it to 3620, the best thing you can do is speak to Dave Winder he was really helpful providing diagrams of how it all went together, the really tricky bit is the hog fitting, but once that is in-place its all about getting the rope lengths right.