I use them often with no problems. Rarely, a customer will say she doesn't want one and prefers copper piping. It's obviously a matter of taste! Most people don't concern themselves much with what goes on under the toilet cistern as long as it works! They've been used in most places on the continent for many years, and certainly do save time.
A technical advantage with a flexi-hose is that it does not subject the plastic inlet tail to any stresses that might make it leak or be difficult to get a watertight joint, especially if the tail is not quite aligned with the connector. Sometimes wc cisterns drop a bit (especially on modern ticky-tacky walls) which can put stresses on a rigid all-copper inlet joint.
A further advantage is that the sealing system provided with the flexi-hoses I use is much more reliable and easy to fit than the usual fibre washer on a rigid connector. This is especially true with the plastic valve inlet tails now common.