Super keen climbers often struggle to find a climbing partner with the same level of enthusiasm or whose days off coincide. In order to put in the necessary mileage to keep fit they can be drawn to self belaying, ie hanging a rope down a route and then climbing the route repeatedly while attached to the rope by a sliding device that will hold them if they fall (they hope!). Some ascenders can also be used to protect a solo climber who is climbing in such a way but few of them are designed with this in mind and unfortunately there is no simple answer as to what is the best device for self top-roping.
The Petzl Shunt has long been favoured by British climbers for self-belaying as it doubles up as a useful device for protecting abseils on either single or double ropes, and also as an ascender, and autobloc, and its smooth method of trapping the rope would suggest that sheath damage is unlikely (and indeed Petzl issue no warning about this). However as a self-protection device it has a serious flaw in that the lever can be inadvertently squeezed on bulges and overhangs meaning that it will not grip the rope if you fall, and also that something (clothing, other gear etc) can get caught in the mechanism also preventing it from working (we know of several ground-falls that have resulted, causing serious injury – one of the best known was to a certain Mr Ron Fawcett – so be warned!). A good method of preventing this from happening is to tie a knot in the rope below you after several metres (allow for rope stretch) so that if you do slip you will not deck it, but this may not be easy to do if you have weighted the end of your rope down with your rucksack. Petzl themselves, although agreeing that the Shunt can be used in this way, issue Dire Warnings with it. See also Shunt - specific instructions.
Having said that, they also issue fairly dire warnings with the Petzl Basic Ascender which is what they used to prefer people to use: Ascender - specific instructions, not the least worrying is the one saying that the sheath may be damaged (by its spiky teeth) with a force of 4.7kN in a Factor 1 fall - realistically though, if you use the Basic as recommended this is unlikely to happen as long as you don't allow any slack to build up in the system as you will simply allow your weight to come on the rope rather than fall on it (hence the need to tie a rucksack on to the bottom of your rope to keep it taut). However it is important to note that all this refers to the Old Basic Model (as pictured). The New Basic Model is no longer considered suitable – see the Petzl Website for a list of potential self-belaying devices and why they can or should not be used.
Another device Petzl make that they recommend for self belaying is the Petzl Mini Traxion. This is neater than a Basic, but works in a similar way with spiky teeth so can still cause rope damage. It has now been replaced by the Petzl Micro Traxion – the same warnings apply.
The only device we found that we felt was better than this is something that was once called the Troll Rocker but is now marketed as the SAR Rocker. Unfortunately these would cost around £80 each at retail so they are not likely to sell against a Shunt. We did use to sell a few when they were £40. The advantage over a Shunt is that the lever is internal so it can't depress inadvertently, nor can things catch on it. However it will only take one rope, and can't be used for much else.
Finally mention should be made of two devices that used to be manufactured by Wren Industries in the States, the Soloist, and the Silent Partner, both of which sound ideal for solo top-topers. Unfortunately they are not available in Europe as they haven't been CEN tested, also they are expensive, but we do know of people who have mail ordered them from the USA. The Silent Partner is now made and sold by Rock Exotica; it is also thought that they own the rights to the Soloist though it does not currently feature on their website.
It is also worth using a karabiner that ensures the pull is along the axis (such as the DMM Belay Master or any other specialised Belay Karabiner) to stop cross-loading, though Petzl say that it is essential to use an oval karabiner with a Mini Traxion.
Be warned, none of the above methods is foolproof. There is a good article on Solo Toproping – Basic Self-Belaying Techniques on Climbing and another good one on the Petzl website called Self-belay for Solo Climbing with a Fixed Belay Rope in which they conclude that it's best to use two devices in tandem.
Ultimately there is unlikely to ever be a completely safe sure-fire cheap device for self top-roping, and in the end, most people who try it give it up, discovering that, however appealing in theory, in practice it lacks that vital something, for as Colin Kirkus once said to Alf Bridge on the summit of Sgurr Alasdair:
"You know Alf, going to the right place at the right time,
with the right people is all that really matters.
What one does is purely incidental."