As a general rule in life you do get what you pay for. Branded chargers will always be better than that cheap charger you got with that £5 dock :P
Sometimes it may not be bad for the device, but cheaper ones tend to be less efficient, so 70% rather than 85%, which equates to a higher electricity bill (not really noticeable with phone chargers though as draw is so small) and may give off interference as they are badly shielded.
So far I've not had any issues using either cheap or branded chargers, but I do tend to go with the branded stuff if I have one to use.
edit: also, slower charging *generally* means longer battery life. i think it's an exponential thing though, so 500mA charge is will not give twice the battery life of 1000mA (1amp) charges. not sure where i picked this up from though but i'm sure you'll find plenty of people agreeing or contradicting things like this