Over the last few weeks, I've had to climb the mast of my sailboat to get at some things that were broken, and also to install a new system to control my mainsail.  I'm afraid of heights, and so doing this was no small task.
A few years ago I bought a system that ran up the track in the mast and it had foot loops in it.  It came with a belt to hold you around the mast as you went up, sort of line a lineman's toolbelt.  It really didn't work all that well.  The foot loops were amazingly uncomfortable – very little support in them.  Your feet would get stuck in them since they flexed so much, and that wasn't a good thing since you would be fighting getting your foot in and out of the loop.  Add to that the somewhat supportive belt, but still requires another rope connected to it for you to go up.
A friend of mine tried to go up, and he's not afraid of heights, and he didn't want to go all the way up on this thing.  I made it up to the first set of spreaders on my boat, and gave up.
About 6 months ago, I saw an ad in some sailing magazine for a product called the ATN Topclimber.  It is a different concept – rig a static line, attach this enhanced bosun's chair and foot loops to it, and up you go.  Oh, and it can be run by a single person, instead of having someone hoist you up, etc.   Obviously the more folks, the safer, but if I had something I needed to do and didn't want to coordinate a whole party of folks…
I ended up purchasing one, along with a good length of high quality line as recommended (1/2").  Many folks recommend just going out and purchasing climbing equipment, as it's likely cheaper, and very similar.  I opted to purchase this since it was designed for this purpose to begin with, and didn't have lots of extra stuff to contend with or learn.  It also packs up very small in an included bag, which happens to also turn into a bag to carry items up with you.
I'm happy to say I've been up the mast a number of times, and even all the way to the top, which I never thought I'd get to.  It's so easy to just work your way up, and at least for folks who are afraid of heights like me, I found that looking out, and not down, while you're going up, makes it much easier.  It also, in some weird way, is much more comforting to not be attached directly to the mast itself – having the static line very tight is key.  I've also found that having the static line angled away from the mast is key so you don't bump into it on the way up.  Plan your angle to come within 1 foot of where you want to stop to work.
The other big benefit of this system is being able to go up on your own, say in an emergency.  I don't know if I would do that, especially in a dire emergency.  I'm not one to be hanging around on the mast with a big storm going, unless it's that or death I guess…  Regardless, it's compact enough that you could take it on a trip away from your normal marina, and if something happened, like a lost halyard or clipped VHF antenna, you could go up and take care of business without having to wait to go back to the dock.
It's still somewhat disconcerting to be at the top of the mast, but it's much safer feeling being strapped totally into the chair, and having the foot straps that you can push off of as well to gain more strength/force, as well as move around.
The only thing I think could be improved are the instructions.  The only details are a single laminated card, and the pictures are very hard to see and understand.  It's a pretty obvious setup, and I didn't have any problems, but it would be nice to see the pictures larger.
Overall, I'm extremely happy and I think it's well worth the money.  I saved already having to pay some local marine folks to climb my mast and fix three things!