generally you would only need hid replacements for your dipped beam, as these are the lights you'll use most of the time whilst driving.
basically it'll improve your brightness and distance you can see. you must have seen a merc/porsch with hids coming up behind you, and their light illuminates further ahead and brigter than your own lights, even though they're a couple hundred metres behind you. and yet they don't blind (not supposed to anyway) oncoming traffic as it's only dipped beam.
my worry isn't whether hid kits are worthwhile, I think they do what it says on the tin, i'm more concerned about the quality of these cheaper kits. which usually go for £200-500 in UK, and manufacturers ask for £1k+ as an optional extra.