I decided to use Bristol Finish - Amber on my spruce mast two years ago. Here in NC the sun is pretty intense (high UV) for much of the year. I also have all external halyards and wanted something more resistant to abrasion than regular spar varnish.
I looked at a number of products and decided to try Bristol Finish. I spoke to a number of folks and most reported great results but a few indicated it was the worst thing they had ever done to their boats.
I'm not sure why they had bad experiences. I can say from first hand experience it has been exceptional on my mast. At 2 years it looks like the day I applied it and I see few signs of any problems. I plan to apply a few maintenance coats this fall as a matter of principle and to ensure I don't get behind it.
I'm in the process of redoing my toe rails and plan to use the Bristol Finish there as well.
The application instructions are pretty clear about the need to apply the proper number of coats. In the case of my mast we applied 10 coats but that is easy to do as you can apply multiple coats in fairly rapid succession and it builds fast. I did 4 quick coats then allowed a few days to harden and a light sanding then 2 coats, sanded, 2 coats, sanded 1 coat, final light sanding and final coat.
I applied it during a terribly hot July in 2006 and used a bit of their Tropical Reducer to aid in application.
I plan to wood and refinish my staysail boom and main boom this fall as well and will apply Bristol Finish there also.
Be sure to read the materials and unless they have changed things you need to use the Amber product as that is the one with the UV protection.
Be sure to wear the proper mask as the vapors are pretty potent.
Good luck, I hope this helps. I can't say how well or how bad the stuff is for anyone else but it sure has impressed me on the mast. I expected maybe a year but I have been pleasantly surprised.