I have used holidayautos several times and the reservation process has worked fine every time, even with extras such as child seats. Also the prices has consistently been the best I could find on line.
However, some of the companies they work with (especially goldcar) are not that great. You get the feeling that they gave holidayautos to good of a deal so that they have to get that money back elsewhere. Be extremely careful when reading conditions and when examening previous damages. Last time (at goldcar in Madeira) they wanted to charge me several hundred euros for a minimal dent on the tires that didn´t show up on my copy. Fortunately I demanded to see the original and on that paper it was noted. Holidayautos sell a rather expensive "full coverage" insurance that might be a good idé. At least it was alot cheaper that the one they sold at the counter. (150 euro at the counter and 65 on line)