One of the big UK sites (not one mentioned here) does have hidden charges in monthly fees and insurance as extra. There are a few though that only charge monthly fees, but they will seriously stick you if the bag is damaged.
So it depends on whether you want to pay extra for insurance or take a risk.
My opinion is that it makes more sense to just save up for the bag. Think about it...you want a bag that costs £750 and it costs £100 a month to rent (with all extra fees), just save up and you will own the bag and that bag will have resale value when you get bored. If you can afford the high monthly fees, then you can afford to tuck some money away each month. Of course in the sales you can buy these bags soooo cheaply or even at a consignment shop. There is one in The States that charges $40 shipping and has loads of bags!
It seems like a lot to pay to rent something as it can run over 10% of the bags cost per month. That is a higher pencentage to pay then renting a car or a flat!! I think that's shocking!
I suppose if you get bored easily, it is an option.
Neither would I in a million years, but I know some women do. If it is for a one-off occasion then it is a perfect situation, as long as you are careful with the bag. I know some sites do weekly rental as well.
My sister uses the bag hire service and gets a new one every few months, but she has spent thousands and doesn't even own a bag. That is all I was trying to say.