METRONOME!!!
After I got my trusty metronome, my playing improved from the first minute. I realized I had been doing something lazy all along - I would speed up certain notes in a bar to compensate for the difficult chord change that occurs at the end. This makes sense as it stops you from falling behind. BUT it makes you lazy since you will not try improve your chord changes (or hand or finger position changes etc...). This was very subtle but noticeable. Now, playing quarter or eighth notes in time forces you to do quick chord changes again.
And yes, the beeps are really annoying and eventually get imprinted on your brain. You even hear them in your sleep. You start singing beep-beep-beep-beep in the shower instead of House of the rising sun. You even have to set it back to 60bpm every night or your heart will keep going at 200bpm! But it is all worth it...
BTW, here is the one I got, the Korg MA-30. Feels like I payed a lot for it, but not sure what these things go for.