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I find I'm much more in tune with the precision of my playing if I'm playing with a metronome. When I used to practice with my drum machine I found I wasn't able to be as precise and nitpicking as I would like in regards to getting everything as tight and clean as possible. The drum machine tended to blur things a bit for me I guess. Just a heads up with an issue you *might* run into. I'm not sure if its a common one or not.
EDIT: On another note dont practice things with a lot of distortion. It can make you think you're a bit better than you are by hiding mistakes, noises, etc. Find a slightly overdriven sound and that'll give you a bit of room to play while still letting you hear your deficiencies. I think this is overlooked quite a bit.