When I travel with my family, which includes 2 young children, apartments are nearly essential in Europe, since European hotels are far less likely to have connecting rooms or offer suite upgrades (I have SPG platinum status) that allow all 4 of us to occupy a single suite but not the same bedroom. And as others observe, apartments are usually much cheaper than comparable hotel rooms.
I understand what a few posters above are saying about never paying in advance except by credit card, but as I understand it (from disinterested parties) in Europe an individual renting on his own often may find wire transfers as a fairly standard way for him to accept money (and seems to be more expected by European customers than is the case with US customrs), while accepting a credit card deposit payment may be a very costly or complicated thing for him. So it is not necessarily evidence that you're being scammed if a renter asks for a wire transfer for a deposit to hold the reservation. I have rented apartments in Europe on two separate occasions where a money transfer advance deposit was required, and had no problems with either rental. A third time I asked the apt. owner if I could pay by credit card since the money transfer fee is so high from the US, and he said, "Just forget the deposit, I'll trust you." That's not to say there isn't more risk with a wire transfer - there is. I'm just saying it is a reasonable thing for a European property owner to request, and not necessarily a scam.
The one thing that I occasionally wish I could find regarding European apartment rentals is a service that would facilitate rentals of fewer than 3 nights. I understand the overhead expense of cleaning up, etc., and I understand that accepting a 1-night rental could cost the owner a 7-night rental. So I don't blame any owner for a 3-night or even 7-night minimum. I just wish there were a way to rent for 1 or 2 nights sometimes, even if I did have to pay a premium.
Finally, prospective apartment renters should start becoming aware of a trend that is affecting vacation rentals in Manhattan and Paris, and seems likely to spread to other cities. Because congested cities like New York and Paris struggle to house their permanent residents, they are beginning to restrict the rights of apartment owners to make short term leases to tourists. In Paris, the law has evidently long forbidden leases of less than 3 years, I believe, and while this law has been poorly enforced, housing authorities there are reportedly beginning to look more closely at short-term leases. In Manhattan, new laws have just been passed this year and go into effect, I believe, in 2011. It would make sense to assume that other crowded cities are at least looking at similar actions.
Who knew I was living illegally in Paris all those times? It won't stop me from renting apartments in the future, if I can find them. But it will make me think more carefully about backup plans, in case I arrive to find that that my best laid plans have "gang agley."