I'm sure JB will be able to be more authoritative on this, but a Shunt is OK as a back-up device but has some major limitations. The first is that it will lock if you tow it down with you as you descend, so you either have to continually move it down by stopping your descent to free a hand, or better you use old school rope access technique (this involves a thin piece of cord tied into the cam, which is trapped in the fingers and is intended to release if pulled firmly in the event of an actual fall or main line failure). The advantage of the second method is the shunt is always high above you, which is important, because the shunt will slide at fairly low loads, so if you fall onto it with a slack lanyard you can go a fair way before stopping. Second big limitation is the pressed metal body is weak, if you hit a knot at speed it may well open the device up causing it to release. Hence old school rope access is all about keeping the shunt high and the lanyard tight.
The ASAP linked can pretty much be left to do its own thing, it will lock if the device runs down the rope above a particular speed, so is much easier to use. Ropeman would be an absolute disaster, toothed ascenders destroy ropes when shock loaded.
Really, it depends what you're needing this for - a one-off bit of work for yourself, or a recurrent or paid job? Either of the latter and I'd avoid doing anything without the right kit and some training. Life's to good to do yourself in at work!
