One of the hard things in espresso preparation is adjusting the dose and grind based on taste. I found this, at least for me, to be hard for quite some time.
But, after making and tasting thousands of shots, I think that in most cases when we do not like flavor, we are actually under-extracting. So if your shots is excessively bitter or excessively sour, in all likelihood, you are under-extracting and need to grind finer and use lower dose to increase extraction.
But espresso preparation and our taste is not that linear so if that does not work for you go in opposite direction ;-) Increase dose and grind coarser.
Often people ask whether high end espresso machine like GS/3, Synesso or Speedster will make better espresso than some cheaper machine or whether this or that machine is better….
After changing countless machines I conclude that what it comes down to is how often you get exceptional shot of espresso from machine and grinder combination and how much you have to work at it.
On low end of the spectrum, your frequency of exceptional shots is low and work rate is high, you really have to work for them. On high-end your frequency is high and work rate is low, all of these given competent technique which is assumed.
Caveat! No matter how good your gear is there is no low limit to how bad shots you can make with it :-)
You have to be able to dose consistently, pack coffee in basket evenly and dial in your grinder and machine into the range to be able to produce exceptional espresso.
Also note that at some price point you start entering territory of diminishing returns. Meaning, large increase in spending results in unproportionally small improvement in quality of espresso made. For espresso machines I think that point is at about $2500.
Always max out your grinder first before maxing out machine ;-)