I bought the ATN Mast Climber, and found the ascenders to be a poor design. the load is attached to a lever on the cam, at the top of the ascender, causing the line to bend 90 degrees each time the load is applied. Half the each leg stroke was wasted bending and straightening the line. I bought ascenders from REI which have the load attached at the bottom of the ascender, near the exiting line, which don't bend the line at all as it is used. Gives twice the climb for each leg extension, and doesn't look nearly as hard on the line.
I initially thought I would use the ATN ascenders with my bosun chair, so didn't buy their harness, but I didn't feel secure enough in the bosun chair, so I bought a climbing harness while at REI. It came in two sizes, and the salesperson thought I looked like I needed the larger, but I should have gotten the smaller as I have to adjust all the straps to their limit to feel at all secure. I am 70 and 205 lbs at 5-11, and can climb my 60' mast without assistance. If alone, I use one of the ATN ascenders on another halyard for safety.
On one climb, a shackle on the foot sling failed and I had to yell for help. Several people on the next dock came running to help but were looking in the water, not up the mast for a victim - kind of funny. First help to get to my boat was the wife of the ex-sailor that lived across the dock from me, but she was afraid to touch the winch to lower me down and her husband wasn't available. Finally a sailor from down the dock a ways arrived, but he seemed nervous as I was telling him which halyard, etc. I had to keep telling him to slow down, relax, and be careful - there's no rush. I was just stuck, not hurt or in any danger - except from him.