Disk brake has never worked properly from new, pads replaced - still rubbish! I've got Shimano hydraulic disc brakes and ever since I've had the bike (from new) the front brake has never worked properly. It used to squeak really loudly and perform badly but I assumed they just needed bedding in (taking the bike back to where I got it from wasn't really an option as I bought it from a shop far from where I live (long story)). After riding around 100 miles on the bike with no improvement I decided to get the pads replaced. The squeaking isn't as loud, but it still does squeak and the performance is still rubbish.

I've read about cleaning the pads and discs with alcohol which I've now done and that hasn't made any difference either so I'm all out of ideas.

It's really annoying as I used to have cable V-brakes which worked brilliantly! My friend is also having trouble with his disc brakes and I'm starting to think disc brakes are more trouble that they're worth?!

Any suggestions would be appreciated!
I suggest first checking that everything is assembled correctly. Make sure the rotor is on the wheel the right way round (it should have arrows in the centre). Download the instructions from Shimano's website and check that everything is in the right place and done up tight (but not too tight!) Or tell us what model you have and ideally post a picture.

Look down at the pads and make sure that they clear the rotor on both sides and that they're parallel to the rotor. Squeeze the lever and make sure they squeeze fairly evenly and with no obvious stiction. If the pads aren't parallelreset the capiler - these directions look fairly useful to me and there's a video (that I haven't watched). Basically, loosen the mounting bolts, squeeze the brake lever, tighten the bolts.