Stopping is so much more important than going! Brake noises such as squealing while stopping, crunching, metal to metal or even a low brake pedal should be fixed as soon as possible. Of course, when your brakes make a continual annoying squeaking or whistling noise, that is likely the “built-in” warning strip that the manufacturer included in the pad. After the pad material has been completely exhausted, the next step would be a relatively loud metal to metal noise.
If your brake pedal seems to be going closer to the floor than it did before, it is probably because it IS closer to the floor. That could mean there is a leak somewhere in the system. If your pedal goes all the way to the floor, you have immediately lost your braking system. Hopefully your emergency brake is still in working order and you can pull on the lever and pump your brakes to stop the vehicle. If the brake pedal feels mushy, that could mean there is air in the system or you are low on brake fluid. In any case, have them checked and then make an informed decision based on facts.
On a rare occasion, one brake line could be rusted through leaving only one working side. The lines go from front driver side to passenger rear and then front passenger to driver rear. This allows the car to stop using the front and rear brakes, rather than left and right side of the car. The real problem here is that if that one last line breaks or rusts through, you have no brakes. You can also have a high hard brake pedal, which could be a sign of a problem with a frozen wheel cylinder, frozen caliper, master cylinder, etc. If your brake pedal feels liked it is pulsating, that is usually related to the rotors. (And note this is different with Anti Lock Braking systems under hard braking). If rotors are within spec, you would only replace the brake pads. If they are out of spec or scored, you would need rotors as well, normally done in pairs. If you hear metal grinding, the caliper is contacting your rotor, and likely will need replacement as well.
If the car pulls to one side while you are braking, it could be due to that caliper, suspension, alignment or uneven brake wear. The caliper clamps down on the brake pad onto the rotor.
Usually brake pads should be replaced around 50,000 miles depending on the type of driving (city vs. highway), driving habits, and material of pads. Also holding your foot over the brake pedal (or two footed driving) can create premature wear of the brake pads.
Most vehicle repairs caught early are less expensive than ones that are deferred. Letting a repair go can generally cause more damage which directly relates to more money flowing out of your pocket. Compare prices but be sure to compare material, manufacturer, & warranty. You get what you pay for!