Logbook loans are targetted at people who can't get conventional loans. Like all such products they charge much higher rates of interest than conventional loans.
In general, these types of loans should be avoided although, in certain circumstances, they can be useful. (e.g. somebody who is unemployed and broke might be offered a secure well-paid job. The only problem is that the firm pays monthly so he won't get paid until 5 weeks after he starts. For such a person, a short-term loan, even at a high interest rate could prove to a real 'life-saver').
To see how these loans work, read the last item posted here:
http://forums.moneysavingexpert.com/showthread.html?t=11 3318
Chris