The fine and prudent sailors who have posted here may not agree with my opinion on this matter, but here goes... In my opinion, ascending the mast is far safer than most people think it is. If you can get two or three strong men or women to haul you up with a sound halyard attached to a sound shackle that his held up by a reasonably sound rig and mast, then you'll be fine. The drive to the marina will likely be more dangerous than the trip up the mast. When I go up, I like to keep my hands and feet loosely about the mast. If there's a failure, then I"m sliding down the to the spreaders or the deck with a belly burn. Once at the masthead, I clip into the spinnaker halyard.
Having said this, you'll need to closely inspect the shackle and halyard before you go up. YOu need to look for any signs of potential failure. I use electrical tape to secure the shackle in the closed position. Have an experienced sailor check your work. I prefer going up on an all-rope halyards tied with to the shackle with a bowline. Be very careful. Once suspended in the chair, bounce around a bit before you go up. Does everything feel solid? Do you feel like you're going to slip out the front? Is the wind blowing too much? Are any boats wakes going to throw you around? If you don't feel comfortable on the way up, then come down and call a professional rigger.
Anyway, I'm never take unnecesary risks when sailing. I'm not unnaturally afraid of heights and I'm not a thrill-seeker. My bottom line is this: I like to know what's going on at the masthead of my boat, which means I"m going up there on a fairly regular basis. I'm always getting the call to send a friend to the the top of their mast, and I'm calling them to send me to the top of mine. After having done this many times, I feel very comfortable with the process.
By the way, WM has a good bosun's chair on sale right now for $150. On a final note, take a digital camera with you so that you can photograph the rigging once you're aloft for later inspection. Good luck and be careful.