I've used these sites many times (US only) and always had a good experience. Rentals are often listed on more than one of these sites (VRBO, Vacation Rentals, Home Away, sometimes even Trip Advisor) so you can compare reviews from different sources. Sometimes the prices are different too, so you can save some money with a bit of research.
Something I find you have to be careful of on these sites are rentals that are listed as private but are actually only semi-private (for example a main house and a guest cottage which are rented out separately).