I should have been practicing with a Metronome the whole time. I feel like most of 20+ years of playing has been wasted! Okay, that is a bit much, but I certainly did not get the most out of all those hours of practice.
This has been my #1 purchase for improving my playing. I highly recommend getting one. If you want to use a phone app, or computer, or drum machine, or play along to the tracks, that is fine, but it will be very beneficial to have something.
I like this one because it is basic, plenty loud, and even has an output jack for headphones or line-in.