I have heard it from numerious people on here to just suck it up and learn how to use a keyboard and mouse, and that a controller will never compare.
Yep, this is a universal truth about gaming with PCs, the superiority of the mouse and keyboard for the vast number of games is something only well understood by people capable of using them. Taking the plunge to learn is good, it'll lower your ability in the short term, but in the long run it'll free you.
The games I have played with keyboard and mouse have been very basic, so my skills arent the best. Im usually fine with WASD, and can move around fine, but when it comes to putting moves together i start to suck. (jumping and crouching through windows in half-life, sprinting and hiding behind walls, executing commands. I basically dont like to let my fingers off the same spots.
Dexterity takes a little while to develop, one thing you want to also watch for is mapping certain keys that make important combos hard, using the little finger on things like ctrl for duck and shift for walking is often preferred for example, because it doesn't impede your ability to move with WASD at the same time, consider what fingers you lose off WASD for different interactions when mapping keys, ideally you want to be able to use all functions on the move.
I also considered buying a gaming mouse with a bunch of buttons on it, but would this be considered kind of cheap? Would hardcore gamers scoff at the idea of having a bunch of actions tied to mouse buttons?
Some might scoff but generally speaking pro gamers have respect for skills so it's really just a case of use whatever gets the job done the best and let your skill speak for themselves. The only 1 warning I would give about mouse buttons is that with high sensitivity, is that moving your fingers to press side or middle mouse buttons can put your aim off a little. I sometimes map things that naturally interrupt firing/aiming on the mouse, such as reload, changing fire modes etc, and avoid mapping things like 2ndry fire. You don't want to take aim correctly and then press say middle mouse button to fire the under-barrelled grenades on a machine gun only for the action of using middle mouse button to set your aim off.
How long until I can expect to really get used to K+B? I would like eventually get good enough where i can play multiplayer and be competitive, but right now I get dominated pretty bad whenever i play any PC game online. (im decent at nexuiz, but i might be playing against people even worse than myself - so who knows)
It's different for each person, depending on how frequently you play and other factors and also how much you have to unlearn bad habits from control pad gaming such as strafing your shots over people rather than purely aiming (common for analogue stick users).
What I would say is that it's a skill that has a much lower ceiling on ability, you reach a maximum with controllers really fast and then you're limited by the control scheme, gamers with m/kb can play for decades and still be improving.
However long it takes is WELL WORTH IT, you'll be better than with a control pad inside a year easily, and you'll be able to school control pad users quickly after then but it'll just keep getting better the more you play.